Rever the strmem() addition and add a TODO element

strmem() was not very well thought out. The thing is the following:
If the string contains a zero character, we want to match it, and not
stop right there in place.

The "real" solution is to use memmem() where needed and replace all
functions that assume zero-terminated-strings from standard input, which
could lead to early string-breakoffs.
This requires a strict tracking of string lengths.

libutil/strmem.c

@@ -1,23 +0,0 @@
-/* See LICENSE file for copyright and license details. */
-#include <stddef.h>
-#include <string.h>
-
-char *
-strmem(char *haystack, char *needle, size_t needlelen)
-{
-	size_t i;
-
-	for (i = 0; i < needlelen; i++) {
-		if (haystack[i] == '\0') {
-			return NULL;
-		}
-	}
-
-	for (; haystack[i]; i++) {
-		if (!(memcmp(haystack + i - needlelen, needle, needlelen))) {
-			return (haystack + i - needlelen);
-		}
-	}
-
-	return NULL;
-}