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.

join.c

@@ -225,7 +225,7 @@ makeline(char *s, size_t len)
 		beg = sp;
 
 		if (sep) {
-			if (!(end = strmem(sp, sep, seplen)))
+			if (!(end = utfutf(sp, sep)))
 				eol = 1;
 
 			if (!eol) {