main/dbus: add an OpenRC user service for the login session bus

Seems to work decently well for an initial integration, setup instructions are provided in README.alpine. Basically:

1. Initial OpenRC-user setup without PAM you basically need to run openrc --user from your shell configuration AFAIK
   • Could compile with OpenRC with optional PAM support in the future (note that BusyBox doesn't support PAM though)
2. Enable and start the dbus user service:
   1. rc-update add --user dbus
   2. rc-service --user dbus start
3. Set the DBUS_SESSION_BUS_ADDRESS variable from the user's shell configuration file
   • could also provide an /etc/profile.d/ file for this
   • but this has some caveats and might break existing setups, see below