pf tests: add a test for max-src-conn

Switch tests using pft_ping.py to inetd. Netcat can only accept a single
connection, we need multiple parallel connections to test max-src-conn. Use the
discard service and modify pft_ping.py to use proper port number.

Implement functionality of 3-way handshake test in pft_ping.py. Make send_params
accessible to sniffer, because answers to SYN+ACK packets should be send with
the same parameters as the original SYN packet.

Finally add a test for max-src-conn and overload.

Reviewed by: kp
Differential Revision: https://reviews.freebsd.org/D46798