Use the "--make-pidfile" when command_background is used

The start-stop-daemon "--make-pidfile" option is now used by default when using
command_background, this requires a pidfile to be specified.
Document command_background option.

Reported-by: Giampaolo Tomassoni <giampaolo@tomassoni.biz>
X-Gentoo-Bug: 399165
X-Gentoo-Bug-URL: https://bugs.gentoo.org/399165

sh/runscript.sh.in

@@ -124,7 +124,11 @@ start()
 	local _background=
 	ebegin "Starting ${name:-$RC_SVCNAME}"
 	if yesno "${command_background}"; then
-		_background="--background --pidfile"
+		if [ -z "${pidfile}" ]; then
+			eend 1 "command_background option used but no pidfile specified"
+			return 1
+		fi
+		_background="--background --make-pidfile --pidfile"
 	fi
 	if yesno "$start_inactive"; then
 		local _inactive=false