services: Add usv user service manager alias

If `vsv` void service manager program is on the system, we also point it
to our own user service directory and make it accessible as `usv` user
service manager.

services/usv.sh

@@ -0,0 +1,5 @@
+#!/usr/bin/env sh
+
+if command -v vsv >/dev/null 2>&1; then
+    alias usv='vsv -d $HOME/.local/state/service'
+fi