scan: never record its own database files (closes #115)

When the database lay under a scanned operand, the walk recorded the
database file, its -wal and -shm files and its .lock file like any other
file, so every rescan updated them and report paired them with empty
files. Once the database and the lock file are open, the scan now notes
their device and inode, and the walk passes over a file with one of
them, as an operand or below one, printing no line and counting
nothing. Matching by device and inode holds through a symlinked
directory, where comparing paths would not. A record an earlier scan
left for one of them is deleted as unverified. README's smoke-test note
on keeping the database out of the tree is gone, as it no longer holds.

Judgement call: walkRules, in exclude.go, carries the noted files to the walk.

Model: opus-5-5

README.md

@@ -575,6 +575,15 @@ Rules for the walk:
 - Never descend into a directory named `.zfs` (ZFS snapshot pseudo-dirs; walking
   them would list every file once per snapshot), not even when it is an operand;
   such an operand is dropped the same way.
+- Never record the scan's own database. Once the database and its `.lock` file
+  are open, `scan` notes the device and inode of the database file, its `-wal`
+  and `-shm` files and its `.lock` file, and the walk passes over a file with
+  one of them, however it reaches it: as an operand, below one, or through a
+  symlinked directory. It prints no line for such a file and does not count it
+  as skipped. A record that an earlier scan left for one of them is deleted,
+  like the record of any file the scan no longer verifies. There is no
+  `-journal` file during the walk, because the database is in WAL mode then (see
+  "Database").
 - Filesystem boundaries are crossed by default. With `-x` (long form
   `--one-file-system`, following the GNU `du`/`rsync` convention), never descend
   into a directory on a different filesystem than its `PATH` operand; each
@@ -1161,10 +1170,6 @@ All of the following, run in this directory, must pass:
     ./sfdupes report
     ```
 
-    (The database lives in a temp directory of its own: inside `$d`, the scan
-    would record it, and its empty lock file would join the `empty1`/`empty2`
-    group.)
-
     Expected from the first `report`: `one.bin`/`copy.bin`/`copy2.bin` form one
     group (two dupe rows, `first` is the lexicographically smallest path);
     `t1/f1`/`t2/f1`/`t3/f1` form one group; `t1/sub/f2`/