Changed TCP ping method.

Changed TCP ping method.
- SYN segments pass easily through firewalls and it is replied both when the
  port is open as well as when it is closed, serving better to the purpose of
  checking if the link reaches a given host. Whereas ACK segments do not get
  replied if the other end is closed or has a firewall in the middle.
- A thoroughly discussion regarding SYN x ACK is available at
  http://www.networkuptime.com/nmap/page4-4.shtml
  http://www.networkuptime.com/nmap/page4-5.shtml