bsdgrep(1): Sneak in some man page updates

• The --exclude{,-dir} and --include{,-dir} directives now match GNU behavior of being processed in order and latest matching directive wins

• --label was previously not really documented, and -L and -l did not indicate that --label applied to them

• The flags listed as being extensions to POSIX spec were not updated with the removal of compression-related flags

MFC after: 1 week