ICMP - full support

[HellFelix]

ICMP has been integrated to work so that the server can be pinged. This should be expanded to allow clients to send echo requests (and other kinds of ICMP traffic) to remote hosts through the tunnel.

[HellFelix]

This is pretty much fixed with 05b2c82.

ICMP is now forwarded when not addressed to the server address. When it is addressed to the server address, the server can answer with an echo reply, but it will not handle any other ICMP traffic.

There are some other types I would like to add support for in the future, namely types 3 (Destination unreachable) and 11 (Time Exceeded), and possibly 5 (Redirect). These are, however, very low on my list of priorities for this project and will probably not be included in the first release of VEIL.