Fix misuses of NULL when talking about the NUL character

Fix misuses of NULL when talking about the NUL character

* lisp/subr.el (inhibit-null-byte-detection): Make it an obsolete alias.

* src/coding.c (setup_coding_system): Use new name.
(detect_coding): Rename null_byte_found => nul_byte_found.
(detect_coding_system): Use new name.
Rename null_byte_found => nul_byte_found.
(Fdefine_coding_system_internal): Use new name.
(syms_of_coding): Rename inhibit-null-byte-detection to
inhibit-nul-byte-detection.
* src/w16select.c (get_clipboard_data): null_char => nul_char.
* src/json.c (check_string_without_embedded_nuls): Rename from
check_string_without_embedded_nulls.
(Fjson_parse_string): Adjust accordingly.
* src/coding.h (enum define_coding_undecided_arg_index)
(enum coding_attr_index): ...null_byte... => ...nul_byte....
* lisp/info.el (info-insert-file-contents, Info-insert-dir):
* lisp/international/mule.el (define-coding-system):
* lisp/vc/vc-git.el (vc-git--call):
* doc/lispref/nonascii.texi (Lisp and Coding Systems): Use the new name.