Make the default start, stop and status functions overridable

This will make it possible to add support for supervision suites such as
runit and s6.

etc/rc.conf.in

@@ -77,6 +77,12 @@
 #rc_crashed_stop=NO
 #rc_crashed_start=YES
 
+# Set rc_supervisor to use a program to monitor your daemons and restart
+# them when they crash.
+# Leaving this undefined uses start-stop-daemon, which is OpenRC's
+# default.
+#rc_supervisor=""
+
 # Set rc_nocolor to yes if you do not want colors displayed in OpenRC
 # output.
 #rc_nocolor=NO