Replace 4 duplicate definitions of _memmem() with a faster implementation
in misc.c. I believe the old code was O(M*N) while the new is "somewhat
like O(M+N) for most cases" yet is shorter. Also, it is declared so that
it's inlined where applicable.

#include "misc.h"
void *jtr_memmem(const void *haystack, size_t hlen,
                 const void *needle, size_t nlen);