usr.sbin: add tcpsso

tcpsso is a command line tool to apply a socket option to an
existing TCP endpoint, which is identified by the inp_gencnt.
tcpsso can be used, for example, to switch the congestion control
module or the TCP stack.

Reviewed by: rrs, rscheff, debdrup, pau amma
Relnotes: yes
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D34139