tccpp: alternative fix for #include_next infinite loop bug

tccpp: alternative fix for #include_next infinite loop bug

This replaces commit 3d409b08893873b917ccb8c34398bc41a4e84d7c

- revert old fix in libtcc.c
- #include_next: look up the file in the include stack to see
  if it is already included.
Also:
- streamline include code
- remove 'type' from struct CachedInclude (obsolete because we check
  full filename anyway)
- remove inc_type & inc_filename from struct Bufferedfile (obsolete)
- fix bug with TOK_FLAG_ENDIF not being reset
- unrelated: get rid of an 'variable potentially uninitialized' warning