Changes to FFIGEN over time.

960213 / lth
- Fixed error in chez.w:basic-type-name where the entry for unsigned-short 
  was a list and not a pair, causing a crash.
- Fixed error in chez.w:dump-struct/union-def to avoid generating code
  for structures and unions with no fields (declarations, effectively).
- Fixed portability problems in process.sch: reverse! changed to reverse.
- Implemented function pointer casts in chez.w:c-cast-expression.
- Changed 'ffigen' driver program to use /usr/include by default, and 
  also to define -D_NO_LONGLONG by default.