tcp: tcp: allow SOL_SOCKET-level socket options via sysctl interface

When using the sysctl interface for setting a SOL_SOCKET-level socket
option, the TCP handler refers to the IP handler, which only handles
SO_SETFIB and SO_MAX_PACING_RATE.
So call sosetopt(), which handles all SOL_SOCKET-level options.
Then you can use tcpsso with SOL_SOCKET-level socket options as
expected.

Reported by: rscheff
Reviewed by: glebius, rscheff
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D42985

(cherry picked from commit bed7633b108930e9e9d2478c75556035938d4e88)