Empty elements in EMACSLOADPATH now stand for the default

Empty elements in EMACSLOADPATH now stand for the default

* src/lread.c (load_path_check): Take path to check as argument.
(load_path_default): New, split from init_lread.
(init_lread): Move calc of default load-path to load_path_default.
Empty elements in EMACSLOADPATH now stand for the default.
(load-path): Doc fix.

* src/emacs.c (decode_env_path): Add option to treat empty elements
as nil rather than ".".

* src/callproc.c (init_callproc_1, init_callproc):
* src/image.c (Vx_bitmap_file_path):
* src/lisp.h (decode_env_path):
* lread.c (Vsource_directory):
Update for new argument spec of decode_env_path.

* leim/Makefile.in (RUN_EMACS): Empty EMACSLOADPATH rather than unsetting.

* lisp/Makefile.in (emacs): Empty EMACSLOADPATH rather than unsetting.

* test/automated/Makefile.in (emacs):
Empty EMACSLOADPATH rather than unsetting.

* doc/emacs/cmdargs.texi (General Variables):
Empty elements in EMACSLOADPATH now mean the default load-path.

* doc/lispref/loading.texi (Library Search):
Empty elements in EMACSLOADPATH now mean the default load-path.

* etc/NEWS: Mention this.

Fixes: debbugs:12100