Detect function-pointer `done' being set twice in a row.

Detect function-pointer `done' being set twice in a row.

Make `done' a function that uses a file-static function pointer, and add
set_done() to alter it.  That wants either the old or new value to be
exit(3).  If it's not, it still alters the stored pointer, to maintain
the existing behaviour, but emits a warning on stderr.  mhbuild(1)
triggers that warning, making this hunch worthwhile.