sockstat: Add a -f option to show FIB numbers of sockets

Reviewed by: zlei
MFC after: 1 week
Sponsored by: Klara, Inc.
Sponsored by: Stormshield
Differential Revision: https://reviews.freebsd.org/D47316

(cherry picked from commit a471d2b401c9b2d3a95d9139c9a6b123f686a8e9)