sysbeep: Adjust interface to take a duration as a sbt

Change the 'period' argument to 'duration' and change its type to
sbintime_t so we can more easily express different durations.

Reviewed by: tsoome, glebius
Differential Revision: https://reviews.freebsd.org/D32619

(cherry picked from commit 072d5b98c4318e20248a6fbea4a5ca7c96992cac)