Improve error messages that impact newcomers

[AltraMayor]

Discussion #704 highlights 3 situations in which Gatekeeper can provide better error messages to help newcomers:

1. Reporting that DPDK does not have a driver for a given NIC. The message should add the link to the list of supported NICs;
2. Checking that the intersection between variables front_ports and back_ports is empty. If not empty, report the problem and direct users to the question Why do Gatekeeper servers need two network interfaces? on our FAQ.
3. Enforcing variable back_mtu to be greater than variable front_mtu so no encapsulated packets to the Grantor servers are dropped. The test should only be performed when running as a Gatekeeper server.

While item 1 has to be done in the C code, items 2 and 3 can be implemented in Lua at lua/net.lua.