Add load-prefer-newer option, to load .el if newer than .elc

Add load-prefer-newer option, to load .el if newer than .elc

* src/lread.c (Fload): Pass load_prefer_newer to openp.
Don't bother checking mtime if openp already did it.
(openp): Add `newer' argument, to check all suffixes
and find the newest file.
(syms_of_lread) <load_prefer_newer>: New option.

* src/callproc.c (call_process):
* src/charset.c (load_charset_map_from_file):
* src/emacs.c (init_cmdargs):
* src/image.c (x_create_bitmap_from_file, x_find_image_file):
* src/lisp.h (openp):
* lread.c (Flocate_file_internal):
* src/process.c (Fformat_network_address):
* src/sound.c (Fplay_sound_internal):
* src/w32.c (check_windows_init_file):
* src/w32proc.c (sys_spawnve): Update for new arg spec of openp.

* lisp/Makefile.in (BYTE_COMPILE_FLAGS): Set load-prefer-newer to t.

* etc/NEWS: Mention this.

Fixes: debbugs:2061