GNU HURD doesn't define PATH_MAX which is referenced unconditionally

in osdep/io.h. This is apparently a common problem: see
http://www.gnu.org/software/hurd/community/gsoc/project_ideas/maxpath.html.

Use an #ifdef to detect whether PATH_MAX is there and if not
arbitrarily use 4096 (like linux). It seems that PATH_MAX properly
shouldn't be used at all, but this should work for now.