init.d/s6-svscan.in: make sure $command exists

While the s6-svscan runscript belongs to OpenRC, the user is required to
install s6 before it can actually be used, potentially leading to
confusion. Check for the existence of $command in start_pre and, if it does not
exist, bail out with an error that makes this observation.

X-Gentoo-Bug: 816978
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=816978

init.d/s6-svscan.in

@@ -21,9 +21,13 @@ depend()
 
 start_pre()
 {
+	if [ ! -e "$command" ]; then
+		eerror "$command is missing (please install s6)"
+	else
 		einfo "Creating s6 scan directory"
 		checkpath -d -m 0755 "$RC_SVCDIR"/s6-scan
-	return $?
+	fi
+	return
 }
 
 stop_post()