Add systemd user services

Ubuntu has been including systemd user services for gnome-keyring since 2016.

Now that GNOME 3.34 includes support for systemd user services, it's worth looking at including these in gnome-keyring directly.

• https://git.launchpad.net/~ubuntu-desktop/ubuntu/+source/gnome-keyring/tree/debian/user
• https://git.launchpad.net/~ubuntu-desktop/ubuntu/+source/gnome-keyring/tree/debian/gnome-keyring.links

cc/ @iainl