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.

That can cause unexpected behavior, especially with third party tools.
The ignored files will be lost if the source tree is re-imported to
another git repository, unless special care is taken.

Whitelist all non-generated makefiles.

To whitelist instfiles/pam.d/xrdp-sesman, add path to all generated
executables listed in the top-level .gitignore, sort .gitignore
alphabetically.

Add mkinstalldir, it's used by Automake on some systems.