hurd: Fix `revoke' symbol exposition from `unlockpt'

`revoke' is MISC only, it should not be exposed along `unlockpt' which is
XOPEN.

	* include/unistd.h (__revoke): New declaration.
	* misc/revoke.c (revoke): Rename to __revoke, and redefine as weak
	alias.
	* sysdeps/mach/hurd/revoke.c (revoke): Likewise.
	* sysdeps/unix/bsd/unlockpt.c (unlockpt): Use __revoke instead of
	revoke.