pathmax: Leave PATH_MAX undefined on the Hurd, and a constant otherwise.

pathmax: Leave PATH_MAX undefined on the Hurd, and a constant otherwise.

* lib/pathmax.h (PATH_MAX): Leave it undefined on GNU/Hurd.
* lib/chdir-long.h: Include pathmax.h.
* lib/clean-temp.c (PATH_MAX): Remove code that is done by pathmax.h.
* lib/getcwd.c: Include pathmax.h instead of <limits.h>.
(PATH_MAX): Remove code that is done by pathmax.h.
* lib/canonicalize.c (PATH_MAX): Provide a fallback value.
* lib/tmpfile.c: Add a comment.
* m4/pathmax.m4 (gl_PATHMAX): Don't test for pathconf.
* modules/chdir-long (Depends-on): Add pathmax.
* modules/getcwd (Depends-on): Add pathmax.
* tests/test-getcwd.c (test_abort_bug): Avoid syntax error when PATH_MAX
is not defined.
* doc/posix-headers/limits.texi: Mention the pathmax module.
* NEWS: Mention the change.