porting dhcp additions to ndpi 4.2 stable

[hl33ta]

4.2 iteration of dhcp additions to ndpi

[shla2022]

Is there a reason ndpi_min was used here, but if (len > ...) checks are used in the rest of the code below? I think the code would be clearer if the same approach was used, either:

if (len > sizeof(buf)) len = sizeof(buf);

or

len = ndpi_min(len, sizeof(buf));

I think both should achieve the same thing, and the code will be easier to read because it will be more consistent.

[shla2022]

Minor suggestion, LGTM.