examples, oneshot services rework

Signed-off-by: fbt <fbt@fleshless.org>

examples/services/rc.local

@@ -0,0 +1,9 @@
+#!/usr/bin/env ssm
+
+service_type = oneshot
+service_command = /etc/rc.local
+
+pre_start() {
+	# Do nothing, successfully, if /etc/rc.local is not executable.
+	[[ -x "/etc/rc.local" ]] || die 0
+}