fork.h: replace with register-atfork.h

UNREGISTER_ATFORK is now defined for all ports in register-atfork.h, so most
previous includes of fork.h actually only need register-atfork.h now, and
cxa_finalize.c does not need an ifdef UNREGISTER_ATFORK any more.

The nptl-specific fork generation counters can then go to pthreadP.h, and
fork.h be removed.

Checked on x86_64-linux-gnu and i686-gnu.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>