syslogd: Stop trying to send remote messages through special sockets

Specifically this was causing the /dev/klog fd and the signal pipe
handling fd to get a sendmsg(2) called on them and always returned
[ENOTSOCK].

r310350 combined these sockets into the main socket list and properly
skipped AF_UNSPEC at the sendmsg(2) call but later in r344739 it was
broken such that these special sockets were no longer excluded since
the AF_UNSPEC check specifically excluded these special sockets. Only
these special sockets have sl_sa = NULL. The sl_family checks should
be redundant now but are left in case of future changes so the intent
is clearer.

MFC after: 2 weeks