Add scan --exclude PATH (closes #163)

The walk neither enters nor records an --exclude path or anything
below it, and prints "scan: skipped PATH (--exclude)" for each one it
reaches. The flag repeats. Each value is resolved to its real path, as
an operand is. The records below an excluded path are kept, as for a
default exclude. An empty value, which would name the working
directory, is a usage error. An --exclude path whose real path scan
cannot find, or an operand at or below one, is a fatal error before
the scan starts. The line naming the database moved out of runScan
into printDatabasePath, which keeps runScan under the lint length
limit.

Model: opus-5-5

README.md

@@ -252,7 +252,7 @@ In order:
   report.
 
 ```
-sfdupes scan [--workers N] [-x] [--no-default-excludes] PATH...
+sfdupes scan [--workers N] [-x] [--no-default-excludes] [--exclude PATH]... PATH...
 sfdupes report > dupes.tsv
 sfdupes trees > dupetrees.tsv
 sfdupes info
@@ -297,6 +297,13 @@ The reference below is also the program's specification.
 - `--no-default-excludes` (`scan`) lets the walk enter the default excludes.
   Those are the system and temporary directories the walk otherwise leaves out,
   such as `/var/tmp` on Linux (see "Rules for the walk").
+- `--exclude PATH` (`scan`) keeps the walk out of `PATH` and everything below
+  it. `PATH` may be a directory or a file. The flag may be given more than once.
+  `PATH` is resolved to its real path, as an operand is. An empty `PATH`, as in
+  `--exclude ""`, is a usage error, reported in one line on stderr with exit 2
+  before anything is scanned. A `PATH` that does not exist, or whose real path
+  `scan` cannot find, is a fatal error. So is an operand at or below one (see
+  "Operands").
 - `--version`, `-v` prints `sfdupes VERSION` (see "Install") to stdout and
   exits 0. It writes nothing to stderr.
 - `--help`, `-h`, alone or after a subcommand, prints the help text to stderr
@@ -355,6 +362,8 @@ default. An operand may be a directory or a regular file.
 - An operand that does not exist is a fatal error, and so is a symlink whose
   target does not exist. `scan` then stops before it walks anything, and exits
   with code 1.
+- An operand at or below an `--exclude` path is a fatal error too. The message
+  names the operand and the `--exclude` path. Both are compared as real paths.
 - Each operand is resolved to its real path before the walk, as `realpath`
   prints it. A relative operand is joined to the working directory as `pwd -P`
   prints it. Every symlink in the path is then followed. A `..` after a symlink
@@ -431,15 +440,16 @@ operands:
   scanned on different schedules into one database.
 - The scan keeps every record at or below a path that the walk leaves out for
   one of these reasons:
+    - it is an `--exclude` path;
     - it is a default exclude;
     - it is not on a persistent filesystem;
     - with `-x`, it is a directory on another filesystem than its operand.
 
     Such records count as outside the scanned operands. So with `-x`, a
     filesystem mounted under an operand can be scanned on its own schedule into
-    the same database. An operand may itself lie at or inside such a path, as
-    "Operands" describes. Its walk verifies the records below it, so they are
-    deleted when their files are gone.
+    the same database. An operand may itself lie at or inside such a path, other
+    than an `--exclude` path (see "Operands"). Its walk verifies the records
+    below it, so they are deleted when their files are gone.
 
 #### Phases
 
@@ -538,6 +548,17 @@ walk practical on large or busy storage pools.
     The path is not counted as skipped. The records below it are kept, as
     described under "What a scan does to the database".
 
+- The walk never enters or records an `--exclude` path, nor anything below it.
+  For each one it reaches, `scan` prints a line on stderr naming it:
+
+    ```
+    scan: skipped /srv/cache (--exclude)
+    ```
+
+    The path is not counted as skipped. The scan keeps the records at and below
+    it, as described under "What a scan does to the database".
+    `--no-default-excludes` does not change this rule.
+
 - Below an operand, the walk does not enter a default exclude. On Linux, the
   default excludes hold temporary files and the kernel's view of the running
   system. They depend on the system, and `sfdupes scan --help` lists those of
@@ -612,7 +633,8 @@ from a thread of its own and never blocks the workers.
   it repeats another or lies under another;
 - the progress display of each phase (see "Progress"). `scan` prints each
   warning for a skipped path as it happens. It prints a line the same way for
-  each path left out as a default exclude or as not on a persistent filesystem;
+  each path left out as an `--exclude` path, as a default exclude or as not on a
+  persistent filesystem;
 - the summary line;
 - a warning when a report still has the database open as the scan ends (see
   "Concurrent reads").
@@ -1179,6 +1201,9 @@ showing the count it stopped at.
 - `1`: a fatal error, a `scan` stopped by `SIGINT` or `SIGTERM`, or a `dwim`
   stopped by either during its scan. A fatal error is one of these:
     - an operand that does not exist, or whose real path `scan` cannot find;
+    - an `--exclude` path that does not exist, or whose real path `scan` cannot
+      find;
+    - an operand at or below an `--exclude` path;
     - a list of mounted filesystems that `scan` cannot read;
     - another `scan` holding the lock (see "The scan lock");
     - a database, its directory or its lock file that cannot be created or
@@ -1198,6 +1223,7 @@ showing the count it stopped at.
       `--workers abc`;
     - `scan` with no operand;
     - `--workers` below 1;
+    - an empty `--exclude` value, as in `--exclude ""`;
     - `report`, `trees`, `info` or `dwim` with a positional argument.
 
 A fatal error is reported on stderr, starting with `sfdupes:`. A failed write to