Add all header files that we need to EXTRA_DIST to ensure they are
actually in the tarball.

Automake is buggy when you mix C and Vala programs.  It tries to call
valac with the full contents of _SOURCES for the program, but valac
errors out when given .h files.  Until this is fixed, we can't list .h
files in _SOURCES or "make dist" breaks.

This also includes code that writes utmp/wtmp records when applicable,
making use the mount infrastructure to detct when those files are
accessible.

Finally, this also introduces a --dump-configuration-items switch.