daemon(8): Don't block SIGTERM during restart delay

I believe this was introduced in the original '-r' commit, r231911 (2012).
At the time, the scope was limited to a 1 second sleep. r332518 (2018)
added '-R', which increased the potential duration of the affected interval
(from 1 to N seconds) by permitting arbitrary restart intervals.

Instead, handle SIGTERM normally during restart-sleep, when the monitored
process is not running, and shut down promptly.

(I noticed this behavior when debugging a child process that exited quickly
under the 'daemon -r -R 30' environment. 'kill <daemonpid>' had no
immediate effect and the monitor process slept until the next restart
attempt. This was annoying.)

Reviewed by: allanjude, imp, markj
Differential Revision: https://reviews.freebsd.org/D20509