* 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.
