Use XRDP_SOCKET_PATH in file_loc.h
Don't define any non-socket paths in file_loc.h, they should come from
the makefiles.
Define all paths unconditionally, they should not be defined elsewhere.
Pass XRDP_SOCKET_PATH as environment variable to the backends.
Discovered in #321. The number of argument for pa_rtpoll_run have
been changed since 6.0.
>=6.0 : int pa_rtpoll_run(pa_rtpoll *f);
<6.0 : int pa_rtpoll_run(pa_rtpoll *f, bool wait);
Check pulseaudio version by PA_CHECK_VERSION macro introduced since
pulseaudio 0.9.16. In case PA_CHECK_VERSION is not defined,
pa_rtpoll_run takes 2 arguments.
