Don't hard code key codes on Haiku

Don't hard code key codes on Haiku

* src/haiku_support.cc (keysym_from_raw_char): New function.
(DispatchMessage): Map raw character to keysym if applicable.
(be_map_key): Delete function.
* src/haiku_support.h (struct haiku_key_event): Update fields
to hold keysym and character instead.
* src/haikuterm.c (haiku_read_socket): Don't call be_map_key,
and use keysym field if provided instead.