start-stop-daemon.sh: complain in start if command is undefined

The default start-stop-daemon start function expects the command
variable to be defined to point to the daemon we want to start.

If the variable is undefined, this means that there will be nothing to
start, and in this case we should complain because it is possible that
the script writer made a typo in the variable name.

sh/start-stop-daemon.sh

@@ -4,7 +4,13 @@
 
 ssd_start()
 {
-	[ -n "$command" ] || return 0
+	if [ -z "$command" ]; then
+		ewarn "The command variable is undefined."
+		ewarn "There is nothing for ${name:-$RC_SVCNAME} to start."
+		ewarn "If this is what you intend, please write a start function."
+		ewarn "This will become a failure in a future release."
+		return 0
+	fi
 
 	local _background=
 	ebegin "Starting ${name:-$RC_SVCNAME}"