unix(4): Add SOL_LOCAL:LOCAL_CREDS_PERSISTENT

This option is intended to be semantically identical to Linux's
SOL_SOCKET:SO_PASSCRED. For now, it is mutually exclusive with the
pre-existing sockopt SOL_LOCAL:LOCAL_CREDS.

Reviewed by: markj (penultimate version)
Differential Revision: https://reviews.freebsd.org/D27011