hurd: make close a cancellation point

and add _nocancel variant.

* sysdeps/mach/hurd/Makefile [io] (sysdep_routines): Add close_nocancel.
* sysdeps/mach/hurd/Versions (libc.GLIBC_PRIVATE, ld.GLIBC_PRIVATE): Add
__close_nocancel.
* sysdeps/mach/hurd/i386/localplt.data (__close_nocancel): Allow PLT.
* sysdeps/mach/hurd/close.c: Include <sysdep-cancel.h>
(__libc_close): Surround _hurd_fd_close with enabling async cancel.
* sysdeps/mach/hurd/close_nocancel.c: New file.
* sysdeps/mach/hurd/not-cancel.h (__close_nocancel): Replace macro with
declaration with hidden proto.