Support equivalents of various netcat options

[jwodder]

Add the ability to configure the following on the command line:

• Use IPv4 only

  • -4, --ipv4
  • tokio::net::TcpSocket::new_v4()

• Use IPv6 only

  • -6, --ipv6
  • tokio::net::TcpSocket::new_v6()

• TCP connection timeout in seconds

• TCP read timeout?

• TCP write timeout?

• Set/unset TCP_NODELAY?

• Size of TCP receive buffer?

  • tokio::net::TcpSocket::set_recv_buffer_size()

• Size of TCP send buffer?

  • tokio::net::TcpSocket::set_send_buffer_size()

• TTL/hop limit of outgoing packets?

  • tokio::net::TcpStream::set_ttl()?

• Initial TCP keep alive timeout in seconds?

• Interval for repeating TCP keep alive timeouts in seconds?

• Number of times to repeat TCP keep alive packets?

• Ask kernel to drop incoming packets whose TTL/hop limit is under a given value?

Some of these values may have to be set using https://docs.rs/socket2.