Don't use `atexit'.

Don't use `atexit'.
* src/files.c (obstack_save): New function.
(done): Rename as...
(output_files): this.
Use `obstack_save'.
* src/main.c (main): Don't use `atexit' to register `done', since
it no longer has to remove tmp files, just call `output_files'
when there are no errors.