listen(2): improve administrator control over logging

As documented in listen.2 manual page, the kernel emits a LOG_DEBUG
syslog message if a socket listen queue overflows. For some appliances,
it may be desirable to change the priority to some higher value
like LOG_INFO while keeping other debugging suppressed.

OTOH there are cases when such overflows are normal and expected.
Then it may be desirable to suppress overflow logging altogether,
so that dmesg buffer is not flooded over long run.

In addition to existing sysctl kern.ipc.sooverinterval,
introduce new sysctl kern.ipc.sooverprio that defaults to 7 (LOG_DEBUG)
to preserve current behavior. It may be changed to any value
in a range of 0..7 for corresponding priority or to -1 to suppress logging.
Document it in the listen.2 manual page.

MFC after: 1 month