evbuffer_find
u_char *evbuffer_find(struct evbuffer *, const u_char *, size_t);
p = evbuffer_find(buf, (u_char*)"\r\n", 2);
p = evbuffer_find(buf, (u_char *)"xy", 2);
p = evbuffer_find(buf, (u_char *)"ax", 2);