* fork.cc (fork_child): Call the __pthread_atforkchild function.

* fork.cc (fork_child): Call the __pthread_atforkchild function.
(fork_parent): Call the __pthread_atforkparent function.
* cygwin.din: Export pthread_atfork.
* thread.h (callback): New class.
(MTinterface): Use it.
* thread.cc (__pthread_atforkprepare): New function.
(__pthread_atforkparent): New function.
(__pthread_atforkchild): New function.
(__pthread_atfork): New function.
* pthread.cc (pthread_atfork): New function.