Remove mallocarray(...) and use reallocarray(NULL, ...)

After a short correspondence with Otto Moerbeek it turned out
mallocarray() is only in the OpenBSD-Kernel, because the kernel-
malloc doesn't have realloc.
Userspace applications should rather use reallocarray with an
explicit NULL-pointer.

Assuming reallocarray() will become available in c-stdlibs in the
next few years, we nip mallocarray() in the bud to allow an easy
transition to a system-provided version when the day comes.

ls.c

@@ -341,7 +341,7 @@ main(int argc, char *argv[])
 	if (argc == 0)
 		*--argv = ".", argc++;
 
-	ents = emallocarray(argc, sizeof(*ents));
+	ents = ereallocarray(NULL, argc, sizeof(*ents));
 
 	for (i = 0; i < argc; i++)
 		mkent(&ents[i], argv[i], 1, Hflag || Lflag);