find: add -xattr and -xttrname

We use -xattr in our openrsync tests for convenience, and it seems like
a good addition to FreeBSD. -xattr and -xattrname will both consult all
available namespaces by default, but -xattrname allows filtering by
namespace using a "user:" or "system:" prefix.

Inspired by: https://github.com/apple-oss-distributions/shell_cmds
Reviewed by: kib, rmacklem
Sponsored by: Klara, Inc.

(cherry picked from commit 22fba3a9d64140d80a9e2093cfc02c9c503b2e19)