MINOR: tcp_sample: implement the fc_saved_syn sample fetch function

MINOR: tcp_sample: implement the fc_saved_syn sample fetch function

This function retrieves the copy of a SYN packet that the system has
kept for us when bind option "tcp-ss" was set to 1 or above. It's
recommended to copy it to a local variable because it will be freed
after being read. It allows to inspect all parts of an incoming SYN
packet, provided that it was preserved (e.g. not possible with SYN
cookies). The doc provides examples of how to use it.