MINOR: net_helper: add ip.fp() to build a simplified fingerprint of a SYN

MINOR: net_helper: add ip.fp() to build a simplified fingerprint of a SYN

Here we collect all the stuff that depends on the sender's settings,
such as TOS, IP version, TTL range, presence of DF bit or IP options,
presence of DATA in the SYN, CWR+ECE flags, TCP header length, wscale,
initial window, mss, as well as the list of TCP extension kinds. It's
obviously fairly limited but can allows to avoid blacklisting certain
valid clients sharing the same IP address as a misbehaving one.

It supports both a short and a long mode depending on the argument.
These can be used with the tcp-ss bind option. The doc was updated
accordingly.