Fix fork() handling for Darwin partially

Fix fork() handling for Darwin partially
(still not handled well for Darwin if GC incremental mode is on)

* os_dep.c (GC_dirty_init): Add FIXME regarding fork (Darwin).
* pthread_support.c (GC_remove_all_threads_but_me): Update mach_thread
for "me" (Darwin).
* pthread_support.c (GC_fork_prepare_proc): Output warning if
GC_dirty_maintained (if DARWIN and MPROTECT_VDB).
* pthread_support.c (GC_fork_prepare_proc, GC_fork_child_proc): Add
FIXME (for Darwin).
* tests/test.c (run_one_test): Do not test fork() for Darwin with
incremental mode on (if HANDLE_FORK); add FIXME.