Mark internal unistd functions hidden in ld.so

Mark internal unistd functions hidden in ld.so

Since internal unistd functions are only used internally in ld.so and
libc.so, they can be made hidden.  __close, __getcwd, __getpid,
__libc_read and __libc_write can't be hidden in ld.so on Hurd since they
will be preempted by the ones in libc.so after bootstrap.

	[BZ #19122]
	* include/unistd.h [IS_IN (rtld)]: Include <dl-unistd.h>.
	* sysdeps/generic/dl-unistd.h: New file.
	* sysdeps/mach/hurd/dl-unistd.h: Likewise.