Fix starting and stopping munin-asyncd. [1]

Turns out creating a start_cmd function is not enough, you have to add a
variable, start_cmd and point it to the start_cmd function.
As for stopping it, it should work out of the box, but for the fact that
munin-asyncd uses setproctitle or something, and it confuses the hell
out of rc.subr.

While there, pet rclint.

PR: 221000 [1] (based on)
Submitted by: marcel herrbischoff com
Sponsored by: Absolight