Move gmx_parse_gpu_id out of checking function

Move gmx_parse_gpu_id out of checking function

Parsing user input should happen separately from running checks for
consistency or sanity. Some of the content of gmx_parse_gpu_id is more
like consistency checking, so left that where it was, using the output
of the parsing to run the checks. Doing so means that the first such
check now works when GMX_GPU_ID is used, when previously it did not.

Made minor improvement to error message text for that check.

Noted TODO to remove helper functionality for GPU sharing checks,
since it is now supported everywhere.

Change-Id: I2ea5841f0fdf461f3024b442a2fe641ea1435f49