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.

libutil/strmem.c

@@ -0,0 +1,23 @@
+/* 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;
+}