[PayasR]

SocketDatagramProtocol and SocketStreamProtocol can now take a socket that's already connected,
replace their socket with a new one, and tell the protocol above them about errors that don't end
the flow. TCP Fast Open, the source address and the end of a stream connection now behave the way
TCP expects.

Bugs this fixes:

• The read and write sources of each socket share a descriptor of their own. A socket that
  replaced another used to inherit the old socket's descriptor while the old sources were still
  cancelling, so nothing watched the new socket and the old one stayed open.
• A stream connection ends at most once. The read and write sides could both report the same
  failure, such as a refused connect, and a write could report the connection as up after it had
  failed.
• With Fast Open, the connection counts as connected once the handshake finishes, so a cancel
  before then doesn't reset it. A writable event before the handshake starts no longer counts as
  its end, and SO_SNDLOWAT stays put until the socket connects.
• A failed SO_REUSEADDR or SO_REUSEPORT fails setup instead of leaving a socket that can't share
  its address.
• The stream protocol's maximumMessageSize no longer traps where Int has 32 bits.

The protocols now:

• Take a connected socket with adoptConnectedSocket(_:). The adopted socket skips the options
  that only shape the connect, such as TCP_CONNECTIONTIMEOUT, and closes when the protocol is done
  with it, even if setup never runs.
• Open a new socket from the same parameters with reset(). The new socket connects from a port
  the kernel picks.
• Report SocketProtocolEvent.softError when a send or receive fails and the flow goes on, and
  softErrorCleared when the next one in the same direction goes through.
• Report connectedLocalEndpoint and connectedInterfaceIndex once connected, and
  maximumMessageSize, receivesSingleMessage and framesAreMessages.
• Connect from the local endpoint's address and port, which connectx binds on Darwin. When the
  TCP options reset the local port, the connection comes from port 0 instead of setting
  SO_REUSEPORT.
• Follow the IP options' local address preference on an IPv6 socket on Darwin, so a flow can
  prefer a temporary or a stable source address.

The stream protocol now:

• Passes the Fast Open connectx flags, sends the first write with the SYN, and holds further
  writes until the handshake finishes. A read before any write starts the handshake, so a peer
  that speaks first still gets a connection.
• Sets TCP_NOTSENT_LOWAT for reduced buffering once connected, and sets TCP_MAXSEG again then.
• Reports a pending SO_ERROR in place of the error from a failed read or write.

The datagram protocol now:

• Sets multicast loopback for a multicast destination, from the IP options, and SO_BROADCAST
  for a broadcast destination. It attaches UDP metadata to the datagrams it receives unless the
  UDP options turn metadata off.
• Reports ENOBUFS as flow controlled, then flow allowed once a send goes through.
• Drops a datagram whose send fails with an ICMP or policy error, and keeps the flow.
• Delivers the datagrams queued with addInitialInboundData(_:ecn:) before anything it reads, and
  counts them in the transfer snapshot.
• Limits maximumMessageSize to the path's largest UDP datagram when that's below 9216 bytes, and
  follows the path in updatePath(_:).

SystemSocket gains init(adopting:), init(rawProtocolFamily:sockType:protocolSubType:nonBlocking:),
boundIPAddress() and setSocketOption(level:name:bytes:). connectSocket and System.connectx take a
source address and connectx flags, and socket creation reports the errno the call returned.