shutdown: add a -q(uiet) flag to suppress warning messages

The system-wide warning makes sense in multi-user environments, but
shutdown(8) may be used on systems or in scenarios where there's nobody
to warn and wall(1) just introduces unnecessary complexity and overhead
to the shutdown process.

Add an option to suppress the warning entirely for those contexts that
want to do so, which are anticipated to mainly be in appliance or
single-user desktop-style systems.

Reviewed by: des
Reviewed by: allanjude, imp (earlier version)
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D46216