ssh: Add systemd ssh@ unit for persistent master connections

These rely on SSH configuring ControlMaster and ControlSocket for the
host so the connection can be reused.

Signed-off-by: Joe Groocock <me@frebib.net>

ssh/askpass

@@ -0,0 +1,13 @@
+#!/bin/sh -e
+
+if [ -z "$SSH_HOST" ]; then
+    >&2 echo 'Missing SSH_HOST variable'
+    exit 1
+fi
+
+case "$SSH_HOST" in
+    *)
+        echo 'Unknown SSH host'
+        exit 1
+        ;;
+esac