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).