allow listening on -bind=address for blocked networks
allow listening on -bind=address for blocked networks

- this allows the client to listen on via -bind specified addresses
  (e.g. 127.0.0.1), even when a network (IPv4 in that case) was blocked
  via e.g -onlynet="Tor"
- introduce enum BindFlags to avoid passing multiple bools to Bind()
- make -bind help text clear we ALWAYS listen on the specified address
- remove an unused variable
- remove 2 unneeded IsLimited() checks before calling Bind(), which does
  these checks anyway

- usage case: specify -bind=127.0.0.1 -onlynet="Tor" to allow incoming
  connections to a Tor hidden service, but still don't allow other IPv4
  nodes to connect / get connected

wastholm/bitcoin,wastholm/bitcoin,wastholm/bitcoin,wastholm/bitcoin,wastholm/bitcoin,wastholm/bitcoin