hurd: make wait4 a cancellation point

and add _nocancel variant.

* sysdeps/mach/hurd/Makefile [io] (sysdep_routines): Add wait4_nocancel.
* sysdeps/mach/hurd/wait4.c: Include <sysdep-cancel.h>
(__wait4): Surround __proc_wait with enabling async cancel, and use
__USEPORT_CANCEL instead of __USEPORT.
* sysdeps/mach/hurd/wait4_nocancel.c: New file, contains previous
implementation of __wait4.
* sysdeps/mach/hurd/not-cancel.h (__waitpid_nocancel): Replace macro with
__wait4_nocancel declaration with hidden proto, and make
__waitpid_nocancel call __wait4_nocancel.