hurd: make open and openat cancellation points

and add _nocancel variants.

* sysdeps/mach/hurd/Makefile [io] (sysdep_routines): Add open_nocancel
openat_nocancel.
* sysdeps/mach/hurd/Versions (libc.GLIBC_PRIVATE, ld.GLIBC_PRIVATE): Add
__open_nocancel.
* sysdeps/mach/hurd/dl-sysdep.c (__open_nocancel): Add alias, check it
is not hidden.
* sysdeps/mach/hurd/i386/localplt.data (__open_nocancel): Allow PLT.
* sysdeps/mach/hurd/not-cancel.h (__open_nocancel, __openat_nocancel:
Replace macros with declarations with hidden proto.
(__open64_nocancel, __openat64_nocancel): Call __open_nocancel and
__openat_nocancel instead of __open64 and __openat64.
* sysdeps/mach/hurd/open.c: Include <sysdep-cancel.h>
(__libc_open): Surround __file_name_lookup with enabling async cancel.
* sysdeps/mach/hurd/openat.c: Likewise.
* sysdeps/mach/hurd/open_nocancel.c,
sysdeps/mach/hurd/openat_nocancel.c: New files.