Add TCP (SYN) support

Instead of sending ICMP ECHO or UDP packets, this mode opens a TCP
connection to the port of choice (80 by default) and sets IP_TTL
or IPV6_UNICAST_HOPS to control the TTL of the outgoing SYN packet.

Instead of using ICMP ECHO sequence or UDP destination port, the
source port number is used to track how many hops away a router is.

For getting the final hop, sockets are left open until a timeout
is reached (10 seconds default) and a write is attempted as soon
as the socket becomes available for writing. Anything other than
a succesful write or a "Connection refused" error is ignored.