[ADD][SSH] ssh-agent service using systemd

I needed to enable the service after linking the package. This might be
better served by yet another step during my Makefile-based installation?
I do not do it enough to warrant adding yet more steps for this
functionality.

ssh/.config/systemd/user/ssh-agent.service

@@ -0,0 +1,10 @@
+[Unit]
+Description=SSH key agent
+
+[Service]
+Type=simple
+Environment=SSH_AUTH_SOCK=%t/ssh-agent.socket
+ExecStart=/usr/bin/ssh-agent -D -a $SSH_AUTH_SOCK
+
+[Install]
+WantedBy=default.target