argp: Distinguish NULL and '\0' from 0.

argp: Distinguish NULL and '\0' from 0.

* lib/argp-pin.c: Include <stddef.h>.
(program_invocation_short_name, program_invocation_name): Use NULL
instead of 0.
* lib/argp-parse.c (convert_options, __argp_parse, __argp_input):
Likewise.
* lib/argp-fmtstream.c (__argp_make_fmtstream): Likewise.
* lib/argp-help.c (make_hol, hol_entry_first_long, hol_find_entry,
hol_append, hol_entry_help, argp_doc, _help): Likewise.
(hol_usage): Use '\0' instead of 0.