Prefer PATH_MAX to MAXPATHLEN

Prefer PATH_MAX to MAXPATHLEN

PATH_MAX is standardized, MAXPATHLEN is not.
Also, the Gnulib pathmax module fixes some rare bugs with PATH_MAX.
So prefer PATH_MAX to MAXPATHLEN unless we know the latter is
also correct (for some platform-specific code).
* admin/merge-gnulib (GNULIB_MODULES): Add pathmax.
This module was already present, as a dependency of canonicalize-lgpl,
but now Emacs is using it directly.  Sort.
* lib-src/emacsclient.c: Include stdint.h, pathmax.h.
(get_current_dir_name): Sync to current src/sysdep.c.
* lib/gnulib.mk.in, m4/gnulib-comp.m4: Regenerate.
* src/sysdep.c: Include pathmax.h.
(get_current_dir_name_or_unreachable):
Use PATH_MAX instead of MAXPATHLEN.