Add detailed network info to getnetworkinfo RPC

This pull adds per-network information to the getnetworkinfo RPC call:

- Is the network limited?
- Is the network reachable
- Which proxy is used for this network, if any

Inspired by #2575.

(After these changes it is also allowed to use `-onlynet=onion` instead of `-onlynet=tor`, as I like it better to call the network `onion`)
Defining const for return-as-value serves no purpose (would be different when returning a reference or pointer).