Stop ns builds polluting the environment with EMACSDATA, EMACSDOC

Stop ns builds polluting the environment with EMACSDATA, EMACSDOC

It's bad form for one part of a program to communicate with another
part by making persistent changes to the environment of all subsequent
child processes.  For example, it can cause odd bugs when building
Emacs from within Emacs (eg bug#6401, maybe).

* nsterm.m (ns_etc_directory): New function, split from ns_init_paths.
(ns_init_paths): Do not set EMACSDATA, EMACSDOC.
* nsterm.h (ns_etc_directory): Add it.
* callproc.c [HAVE_NS]: Include nsterm.h.
(init_callproc_1, init_callproc) [HAVE_NS]: Use ns_etc_directory.