cmp: fix -s (silent) when used with skip offsets

-s causes cmp to print nothing for differing files, for use when only
the exit status is of interest.

-z compares the file size first, for regular files, and fails the
comparison early if they do not match.

Prior to this change -s implied -z as an optimization, but this is not
valid when file offsets are specified. Now, enable the -z optimization
for -s only if both skip arguments are not provided / 0.

Note that using -z with differing skip values will currently always
fail. We may want to compare size1 - skip1 with size2 - skip2 instaead,
and in any case the man page should be clarified.

PR: 252542
Fixes: 3e6902efc802ab57fc4e9bf798f2d271b152e7f9
Reported by: William Ahern
Reviewed by: markj
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D28071