realloc: still more improvements for realloc (p, 0)
* doc/posix-functions/malloc.texi: AIX is sans _LINUX_SOURCE_COMPAT.
* doc/posix-functions/realloc.texi:
Give more details about what recent POSIX versions say,
and fix some misperceptions about C89 through C11.
Document that the GNU behavior does not conform to POSIX,
and that AIX's behavior with _LINUX_SOURCE_COMPAT
matches glibc's circa 1999 behavior, not its current behavior.
Be a little less confident about realloc-gnu, since it doesn't
fully implement current glibc behavior.
Give more details about when memory leaks or false positives can occur.
* doc/posix-functions/reallocarray.texi:
Say that it shares realloc’s woes with zero sizes.