Allow withholding the SYN|ACK packet by user code

[eeeeeta]

In certain use cases, it's desirable to not send a SYN|ACK packet immediately after receiving a SYN -- for example, a TCP proxy that doesn't want to do so until it's connected to the end destination, because the outgoing connection might get refused. (Currently, you have to send a SYN|ACK and then reset the connection afterwards in this case, which is what we do in onionmasq.)

To fix this, add a simple synack_paused flag, controllable by user code, that withholds SYN|ACK packets in SynReceived state until it is unset.

[codecov[bot]]

Codecov Report

Attention: 17 lines in your changes are missing coverage. Please review.

Comparison is base (f3be43a) 79.55% compared to head (21751ec) 79.53%. Report is 5 commits behind head on main.

Files	Patch %	Lines
src/socket/raw.rs	44.44%	5 Missing :warning:
src/socket/udp.rs	44.44%	5 Missing :warning:
src/socket/tcp.rs	55.55%	4 Missing :warning:
src/socket/icmp.rs	70.00%	3 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #863      +/-   ##
==========================================
- Coverage   79.55%   79.53%   -0.02%     
==========================================
  Files          78       78              
  Lines       27917    27896      -21     
==========================================
- Hits        22208    22186      -22     
- Misses       5709     5710       +1     

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[eeeeeta]

Fair enough!