* hurd/hurd/lookup.h: New file.

* hurd/hurd/lookup.h: New file.

	* hurd/hurdlookup.c (__file_name_lookup_under): New function.
	(__hurd_file_name_lookup, __hurd_file_name_lookup_retry,
	__hurd_file_name_split): Rewritten to take callback functions for
	using any needed init or dtable port, instead of passing in crdir
	and cwdir ports.
	(__file_name_lookup, __file_name_split): Use new calling
	convention; pass _hurd_ports_use and __getdport as the callback
	functions.

	* sysdeps/mach/hurd/chroot.c: Use __file_name_lookup_under instead
	of __hurd_file_name_lookup.
	* sysdeps/mach/hurd/chdir.c: Likewise.
	* sysdeps/mach/hurd/fchdir.c: Likewise.
	* hurd/fchroot.c: Likewise.