Support TCP simultaneous open

[lrh2000]

RFC 9293 states:

A TCP implementation MUST support simultaneous open attempts (MUST-10).

It also details the related state machine (Figure 7: Simultaneous Connection Synchronization):

TCP Peer A                                       TCP Peer B

1.  CLOSED                                           CLOSED

2.  SYN-SENT     --> <SEQ=100><CTL=SYN>              ...

3.  SYN-RECEIVED <-- <SEQ=300><CTL=SYN>              <-- SYN-SENT

4.               ... <SEQ=100><CTL=SYN>              --> SYN-RECEIVED

5.  SYN-RECEIVED --> <SEQ=100><ACK=301><CTL=SYN,ACK> ...

6.  ESTABLISHED  <-- <SEQ=300><ACK=101><CTL=SYN,ACK> <-- SYN-RECEIVED

7.               ... <SEQ=100><ACK=301><CTL=SYN,ACK> --> ESTABLISHED

In short:

1. If a TCP socket is in SYN-SENT state and receives a SYN packet (not a SYN|ACK packet), it should change its state to SYN-RECEIVED. This is not previously implemented, and such packets are silently dropped. This PR implements the above behavior.
2. If it then receives an ACK packet acknowledging the SYN packet, it should change its state to ESTABLISHED. This is already implemented.

[whitequark]

I remember reading about simultaneous open in the original TCP RFC and thinking that it's a pretty silly thing that I've never seen used.

[lrh2000]

I remember reading about simultaneous open in the original TCP RFC and thinking that it's a pretty silly thing that I've never seen used.

Yeah, agree. It could be useful for TCP hole-punching, but it is still a very limited scenario. However, per the RFC, this is required to be supported...

By the way, thanks for your review!