Fix detection of WebDAV and Gnutella (over HTTP)
Fix detection of z3950
Add two fuzzers to test `ndpi_memmem()` and `ndpi_strnstr()`
Remove some dead code:
* RTP: the same exact check is performed at the very beginning of the
function
* MQTT: use a better helper to exclude the protocol
* Colletd: `ndpi_hostname_sni_set()` never fails
Update pl7m code (fix a Use-of-uninitialized-value error)
