netcat: Add a flag to enable the use of SO_REUSEPORT_LB

This is handy for testing purposes. Hide it behind a long option so
that it doesn't conflict with upstream, like we do with several other
flags.

Reviewed by: imp, glebius, delphij
MFC after: 2 weeks
Sponsored by: Klara, Inc.
Sponsored by: Stormshield
Differential Revision: https://reviews.freebsd.org/D48543