Currently the `getnetworkinfo` returns something like: 

```
"localservices": "0000000000000449",
```

Manually parsing this consistently gives me a headache (see also https://github.com/sipa/bitcoin-seeder/pull/77).

It would be useful to parse this and show the right combination of `NODE_NETWORK`, `NODE_GETUTXO`, `NODE_BLOOM`, `NODE_WITNESS`, `NODE_NETWORK_LIMITED` and `NODE_COMPACT_FILTERS`. In addition the help text for `getnetworkinfo` could be expanded to mention the right BIP numbers for known flags.

There is some ad hoc parsing to populate e.g. `localrelay`, etc (?).  Their help text could also refer back to `NODE_GETUTXO`.

See also: https://en.bitcoin.it/wiki/Protocol_documentation#Network_address
{
    if (m_fallback.which() == 1) {
        return true;
    } else {
        return RPCArg::Optional::NO != boost::get<RPCArg::Optional>(m_fallback);
    }
}
std::string RPCArg::ToDescriptionString() const
{
    std::string ret;
    ret += "(";
    if (m_type_str.size() != 0) {
        ret += m_type_str.at(1);
    } else {
        switch (m_type) {
        case Type::STR_HEX:
        case Type::STR: {
            ret += "string";
            break;
        }
        case Type::NUM: {
            ret += "numeric";
            break;