hurd getcwd: Allow unknown root directory

To be efficient, the remap translator simply returns ports from the underlying
filesystem, and thus the root directory found through browsing '..' is the
underlying root, not the remap root. This should not be a reason for getcwd to
fail.

* sysdeps/mach/hurd/getcwd.c (_hurd_canonicalize_directory_name_internal): Do
not remove the heading slash if we got an unknown root directory.
(__getcwd): Do not fail with EGRATUITOUS if we got an unknown root directory.