bsnmpd: fix unix/stream socket operation

With new send method, that gives us directly port_input pointer, we know
the correct file descriptor right away, no matter are we in SOCK_DGRAM or
in SOCK_STREAM mode. Previously, the function tried to search for the
socket, and that was totally wrong. With several simultaneous connections
this end up with sending a reply to incorrect socket.

Reviewed by: harti
Differential Revision: https://reviews.freebsd.org/D51361