Implement strmem() and use it in join(1)

We want our delimiters to also contain 0 characters and have them
handled gracefully.
To accomplish this, I wrote a function strmem(), which looks for a
certain, arbitrarily long memory subset in a given string.
memmem() is a GNU extension and forces you to call strlen every time.

join.c

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