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.

util.h

@@ -58,6 +58,8 @@ size_t estrlcpy(char *, const char *, size_t);
 #undef strsep
 char *strsep(char **, const char *);
 
+char *strmem(char *, char *, size_t);
+
 /* regex */
 int enregcomp(int, regex_t *, const char *, int);
 int eregcomp(regex_t *, const char *, int);