getfacl: implement --skip-base (-s)

This skips files that only have a trivial ACL, which is useful for
callers that want to examine files with more unique ACLs.

While we're here, don't issue the file name/ownership header if we're
just going to error out on the file; this adds extra noise for little
gain.

Reviewed by: markj, olce
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D40603