Warn about and skip non-regular and .zfs operands, keeping their records (closes #9)

A symlink, socket, FIFO or device-node operand, or a directory operand
named .zfs, was silently ignored yet stayed in the scanned operands, so
the update phase deleted every record stored beneath it. Such an
operand now gets a one-line warning, counts as skipped, and is dropped
before overlapping operands are pruned and the database index is
loaded: another operand beneath it is still scanned, and the records
beneath it count as outside the scanned operands and are not deleted,
unless it lies under another operand. The exit status stays 0. An
operand that turns into one of these after that check is warned about
and skipped by the walk instead. README "scan mode" and "Rules for the
walk" say so.

Model: opus-5-5

README.md

@@ -278,6 +278,18 @@ duplicates another or lies under another is dropped before walking,
 so every file is reached exactly once and produces one database
 record.
 
+An operand that is a symlink (never followed, not even as an operand),
+socket, FIFO, or device node, or a directory named `.zfs`, is not
+scanned. `scan` prints a one-line warning naming the path and what it
+is, counts it as skipped, and drops it from the scanned operands before
+reading the database. Another operand beneath it is still scanned. The
+records stored beneath it are not deleted: they are treated like any
+other record outside the scanned operands, including the content-phase
+exception below. If it lies under another operand, they are under that
+operand instead, and are deleted like any other record there that this
+scan did not verify. This is not an error: a scan whose every operand
+is dropped walks nothing and exits 0.
+
 `scan` synchronizes the database with the filesystem state under the
 scanned operands:
 
@@ -383,9 +395,12 @@ during the hash phase:
 Rules for the walk:
 
 -   Only regular files. Skip directories, symlinks (do not follow,
-    including symlink operands), sockets, FIFOs, and device nodes.
+    including symlink operands), sockets, FIFOs, and device nodes. An
+    operand that is a symlink, socket, FIFO, or device node is dropped
+    as described in "`scan` mode" above.
 -   Never descend into a directory named `.zfs` (ZFS snapshot pseudo-dirs;
-    walking them would list every file once per snapshot).
+    walking them would list every file once per snapshot), not even
+    when it is an operand; such an operand is dropped the same way.
 -   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
@@ -396,9 +411,11 @@ Rules for the walk:
     path, and continue. Per-file errors never abort the run; the final
     summary reports how many were skipped. As specified above, a
     skipped path that has a database record from an earlier scan loses
-    that record, unless it failed only in the content phase; an
-    unreadable directory subtree likewise loses its records (accepted:
-    the database mirrors what the latest scan could actually verify).
+    that record, unless it failed only in the content phase, or is an
+    operand dropped before the database was read that lies under no
+    other operand; an unreadable directory subtree likewise loses its
+    records (accepted: the database mirrors what the latest scan could
+    actually verify).
 
 Concurrency: the walk phase (which also stats files), the hash phase,
 and the content phase each use a worker pool of `--workers` workers