Add 2 virtual methods to Card to load and save snapshots.
Uthernet card: support save/load state
* Make Uthernet settings behave like all other cards. Fix as well the fact that the Uthernet interface would not be reapplied after a restart. * Uthernet: use consistent types to reduce code complexity. Use std::string everywhere.
* Make tfe easier to compile in Linux. * libpcap in Linux: we can just use the libpcap provided and link to it directly.
