[AD] namespace conflict with curses

[Peter Wang <tjaden@xxxxxxxxxx>]

`hline' and `vline' are in conflict with the curses API.  How should we
fix this?

- We could handle it in the same way as we did for `clear'.  We give new
  names to `hline' and `vline' and put in aliases in alcompat.h, which
  can be switched off by #define ALLEGRO_NO_HVLINE_ALIAS.

- We could do the same as the previous option, but NOT use the gcc
  deprecation warnings.  Neither the old names nor the new names would
  be preferred, but the old names could be disabled.
 
- We could keep hline/vline as the "official" functions (what is listed
  in the documentation), but internally rename them to something like
  allegro_hline.  Again the hline/vline wrappers would have to be
  disable-able.

Peter