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 ignored without a word and stayed in the scanned
operands, so the update phase deleted every record stored beneath it.
Such an operand now gets a one-line warning naming the path and what it
is, counts as skipped in the summary, and is dropped before the
database index is loaded, so 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 between that check and the walk 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)