Merge hardware.pulseaudio and services.pulseaudio

More specifically, this removes services.pulseaudio and adds the option
hardware.pulseaudio.systemWide which defaults to false but can be used to turn
on the system-wide PulseAudio server (previously defined in
services.pulseaudio). Since the two PulseAudio modes are mutually exclusive
anyway (maybe not strictly true, but I don't think is a good idea combining
them) its nicer to be able to reuse server and ALSA configuration between them.

Also the system-wide PulseAudio service has been adjusted to systemd, and a few
things has been fixed (there was no alsa.conf before, for example).

The bottomline is that people that was using hardware.pulseaudio before should
be able to keep doing it in exactly the same way, and people that used
services.pulseaudio must switch over to hardware.pulseaudio.systemWide instead.