Hurd: Implement chdir support in posix_spawn

This fixes build-many-glibcs.py on i686-gnu.

Thanks Florian Weimer for the initial version.

* sysdeps/mach/hurd/spawni.c (__spawni): Add ccwdir port. Test and use
it, free it if needed.
(reauthenticate): Test and use ccwdir.
(child_init_port): In non-resetids case, test and use ccwdir.
(child_chdir): New nested function to set ccwdir.