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

TODO.md

@@ -29,6 +29,10 @@
 
 # Completed Steps
 
+- warn about and skip symlink, socket, FIFO, device and `.zfs`
+  operands, keeping the records beneath them (2026-10-03,
+  https://git.eeqj.de/sneak/sfdupes/issues/9)
+
 - `report` and `trees` open the database read-only, and `scan` leaves it
   out of WAL mode, so reading needs only read access (2026-10-03, closes
   https://git.eeqj.de/sneak/sfdupes/issues/8)