MFH: r448520

Make the provided rc script more robust.

Also add an asterisk_stopsleep knob (disabled by default) to allow
users to work around a possible race condition when asterisk is sent
a stop command just after launching, but before it's startup is
actually completed.

PR: 221271
Submitted by: O. Hartmann <ohartmann@walstatt.org>

Approved by: ports-secteam (delphij)