Base to get all 'gmx help' contents to user guide

Base to get all 'gmx help' contents to user guide

Implement initial functionality to get all 'gmx help' contents to the
reStructuredText user guide.  Make 'gmx help -export rst' export also
the help topics, and make minimal changes to the user guide and the
selection help to get it go through Sphinx cleanly.  Subsequent changes
will clean up the documentation further.

Part of #679.

Change-Id: I66219110fac4f4acd9ff9c36a19ca10f9561ca29