syslogd: Fix handling of unix socket modes

When bind() is called, the process umask is applied, so one has to
either clear the umask before binding or call chmod() to add permissions
after the fact. Do the former here to ensure that the socket always has
the correct mode.

Reported by: Lexi Winter <lexi@le-fay.org>
Fixes: 2b8c3a05e0a6 ("syslogd: Set unix socket modes atomically")