Print common options to 'gmx -h'

The help shown for the wrapper binary now includes the command line
options it accepts.  Remaining issues that will be fixed separately:
 - Unlike other help output, this output is actually influenced by what
   the user specifies on the command line.
 - The information is not so easy to find, because of the long list of
   subcommands that comes after it.
 - The information should go to some man page as well.

Related to #685.

Change-Id: Iac72dd21d4a733016b331549e139d4184bd3e283