pulseaudio nixos module: run as systemd user service instead

Most of the desktop environments will spawn pulseaudio, but we can instead simply run it as a systemd service instead.

This patch also makes the system wide service run in foreground as recommended by the systemd projects and allows it to use sd_notify to signal ready instead of reading a pid written to a file. It is now also restarted on failure.

The user version has been tested with KDE and works fine there.

The system-wide version runs, but I haven't actually used it and upstream does not recommend running in this mode.