Add options for setting the source port and IP address

[jwodder]

• Options:

  • -p/--source-port <port> — Set source port
  • -s/--source-ip <ip> — Set source IP address

• This can be done via tokio::net::TcpSocket::bind().

• Problem: Setting the source IP requires committing to either IPv4 or IPv6 and making a connection to one IP at a time, which means extra work when a domain name has multiple IPs. (Then again, std::net::TcpStream::connect() handles domains with multiple IPs by just trying them one at at time.)