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.

sed.c

@@ -598,7 +598,7 @@ strtorunes(char *s, size_t nrunes)
 {
 	Rune *rs, *rp;
 
-	rp = rs = emallocarray(nrunes + 1, sizeof(*rs));
+	rp = rs = ereallocarray(NULL, nrunes + 1, sizeof(*rs));
 
 	while (nrunes--)
 		s += chartorune(rp++, s);