Fix feature-related compilation issues.

[Dirbaio]

The goal is that every combination of features either:

• compiles cleanly, or
• fails to compile with a human-readable error message

I've opted to make "no ipv4 and no ipv6" an unsupported combination. It could be made to work but I don't think it's worth it.

Fixed cases:

• (no features) -> now fails with You must enable at least one of the following features: proto-ipv4, proto-ipv6. Fixes 0.7 build error #428 (comment)
• proto-ipv4,socket -> now fails with If you enable the socket feature, you must enable at least one of the following features: socket-raw, socket-udp, socket-tcp, socket-icmp. Fixes 0.7 build error #428 (comment)
• ethernet -> now correctly enables socket feature, and then fails with the error above.
• proto-dhcpv4 -> now correctly enables ethernet feature.
• proto-ipv4,phy-tap_interface -> now correctly enables ethernet feature.

fixes #428

Will be cherry-picked for 0.7.1 release

[whitequark]

I've opted to make "no ipv4 and no ipv6" an unsupported combination. It could be made to work but I don't think it's worth it.

I think this is a reasonable decision. That's what I aimed to do anyway, but I think at the time compile_error! was not a thing.