* c-lex.c: s/change_file/file_change.
* cpplib.h: Similarly.
* cppmain.c: Similarly.
* fix-header.c: Similarly.
* cppfiles.c (stack_include_file): Pass the buffer location and
size to cpp_push_buffer. Generate the file_change callback,
so that sysp is already set.
* cpphash.h: Add _cpp_do_file_change.
* cpplib.c (do_line): Set buffer->sysp directly. Generate the
file_change callback after setting sysp.
(_cpp_do_file_change): Handle FC_ENTER and the FC_RENAME exception
here.
(cpp_push_buffer): Don't generate a callback. Clear sysp.
(cpp_pop_buffer): Clean up logic.