find: Allow '/' to be used with -perm for GNU compatibility

In 2005, Gnu find deprecated '+' as the leading character for the -perm
argument, instead preferring '/' with the same meaning. Implement that
behavior here, and document it in the man page.

Reviewed by: imp, emaste
Pull Request: https://github.com/freebsd/freebsd-src/pull/1060

(cherry picked from commit 2a121b97e9673ff37062c9fa026eee969940d2e2)