Fix segfault in 'gmx -h'

Combination of two recent changes caused 'gmx -h' to segfault because of
an uninitialized variable.  This fix is in cmdlinehelpcontext.cpp.

Added a test for this case, which required some extra functionality in
the module manager.  This is all the other changes.

Change-Id: Ie845c287975575c51c42763c03ac99bca047699c