Shell completion export from the wrapper binary

Bash completions now complete the arguments to the wrapper binary
itself, as well as the subcommand names.  After a subcommand has been
specified, use the existing completion logic.  Completions now work also
for arbitrarily suffixed binaries (which the old system didn't work at
all).  No completion is generated for symlinks, but should be
straightforward to do if really required; approach would be the same as
for GMX_BUILD_MDRUN_ONLY.

Other completions are not working for the wrapper binary, and as far as
I can tell, require a completely different approach. Removed the
non-functional code.

Related to #685 and #1410.

Change-Id: I55b13a65c176dab6e2f4f41bf6e829112c99e6b3