PR# 14516 Routine `eif_system_asynchronous' calls fork instead of eif_thread_fork

Routine `eif_system_asynchronous' in misc.c calls `fork' instead of
`eif_thread_fork'.  This can cause problems, as I learned when making eweasel
multithreaded.  From my e-mail relating to this problem:

My theory is that when the thread that was supposed to run the test called fork,
another thread was requesting a GC.  After the fork, only the calling
thread is replicated in the new process.  It then calls a routine, which checks via RTGC
whether another thread is requesting GC (eif_is_gc_collecting).  The answer is yes,
so it calls eif_synchronize_gc.  It then tries to lock eif_gc_mutex but this mutex
is already locked and will never be unlocked since the thread holding the lock was
not replicated by the fork.  So the (only) thread in the child process blocks forever.