systemd --user support
Here's a branch to add systemd user units for g-s-d programs. We need to know when they're ready, so as discussed previously on IRC I made the plugins all own a name on the session bus after they've started up. This lets us avoid linking to libsystemd.
There are a few .target units. I expect session definitions to start gnome-settings-daemon-x11.target or gnome-settings-daemon-wayland.target as appropriate. The wayland one is different from the x11 one in that it orders itself after Shell (for XWayland). The -gdm targets start only the programs we need in the GDM session - and having this lets us remove the hardcoded list in the skeleton.
And then there's a directory that is used to disable the autostart files when we're using systemd --user instead. This lets the current way continue to be used so we don't break third parties (yet).