libc/syslog: deprecate use of "/var/run/logpriv"

This additional socket was created in 2e89951b6f20 and 240d5a9b1ce76
to try workaround problems with classic PF_UNIX/SOCK_DGRAM sockets.

With recent changes in kernel this trick is no longer needed, so the
trick can be reverted.

In syslogd(8) we would still create the socket for the next several
major releases for compatibility.

Differential revision: https://reviews.freebsd.org/D35305