Add support for addition and deletion of layers

Tracebox now maintains the added and suppressed layers. Example:

tracebox to 217.218.26.200 (medu.ir): 64 hops max
...
13: 195.146.33.29 (195.146.33.29) TCP::DataOffset TCP::CheckSum IP::TotalLength IP::TTL IP::CheckSum +TCPOptionMaxSegSize
...

Shows that at hop 13, the MSS was added.

Signed-off-by: Gregory Detal <gregory.detal@uclouvain.be>