socket: Add an option to retrieve a socket's FIB number

The SO_SETFIB option can be used to set a socket's FIB number, but there
is no way to retrieve it. Rename SO_SETFIB to SO_FIB and implement a
handler for it for getsockopt(2).

Reviewed by: glebius
MFC after: 2 weeks
Sponsored by: Klara, Inc.
Sponsored by: Stormshield
Differential Revision: https://reviews.freebsd.org/D48834