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.

col.c

@@ -177,7 +177,7 @@ allocbuf(void)
 {
 	char **bp;
 
-	buff = emallocarray(pagsize, sizeof(*buff));
+	buff = ereallocarray(NULL, pagsize, sizeof(*buff));
 	for (bp = buff; bp < &buff[pagsize]; ++bp)
 		*bp = emalloc(NCOLS);
 }