User friendliness: automatically adjust parameter lua/net.lua:front_mtu

[AltraMayor]

The configuration file lua/net.lua comes with the parameters front_mtu = 1500 and back_mtu = 2048, which is the proper setting for Gatekeeper servers, but not for Grantor servers. Grantor servers do not have a back net, and their front net should have an MTU of 2048.

Passing the parameter gatekeeper_server of lua/main_config.lua to netf() (i.e., the function that lua/net.lua returns) enables lua/net.lua to correctly set the parameters front_mtu accordingly to the role of the server.

Once this feature is implemented, the wiki page Tips for Deployments must be updated to reflect this change.