Clean up gmx_fatal.*

- Remove unused functions and macros.
- Replace some one-use wonders with a direct call to gmx_fatal().
- Add basic Doxygen documentation.
- Clean up some (but not all) gmx_fatal.h includes, removing unnecessary
  ones and adding some transitively covered ones.  Didn't go through all
  the files, and didn't do this very carefully, so some are still left.
  but as long as vec.h includes gmx_fatal.h, it doesn't really matter...

Preparation for moving gmx_fatal.h away from legacyheaders/.

Change-Id: I96934ac6ce6bca1e4e8473bdee2b8fec036b0f76