Detect MTU on macOS and FreeBSD to adaptively reduce discovery packet

Detect the MTU on macOS and FreeBSD (which share the network parts of FreeBSD) and adaptively remove the outgoing and incoming capabilities.
The incoming capabilities are usually shorter, fit the size and help initialize the plugin list on the peer. This should fix an empty plugin list when the identity packet is sent by the macOS app.

If the MTU is still too short, both the capabilities need to be removed, which is not likely to happen.