syslogd: Do not add shutdown sockets to the kqueue

If syslogd is in secure mode, all INET sockets are shutdown. Do not
add these to the kqueue because we can't read from them.

Also, remove the listen() call when setting up sockets. Syslogd
exclusively uses SOCK_DGRAM sockets, so calling listen() is useless.

Reviewed by: markj
MFC after: 3 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D41526