catch EPERM sendmsg errors for the very first packet on Linux

[marten-seemann]

This is a workaround for golang/go#63322. On Linux, we now ignore the EPERM error for the very first sendmsg call on a new connection, and retry exactly once.

This is expected to dramatically reduce the flakiness of our test suite.

[codecov]

Codecov Report

Merging #4111 (86de97c) into master (49e588a) will increase coverage by 0.02%.
Report is 1 commits behind head on master.
The diff coverage is 81.25%.

@@            Coverage Diff             @@
##           master    #4111      +/-   ##
==========================================
+ Coverage   83.71%   83.73%   +0.02%     
==========================================
  Files         150      150              
  Lines       15401    15415      +14     
==========================================
+ Hits        12892    12907      +15     
+ Misses       2010     2009       -1     
  Partials      499      499              

Files	Coverage Δ
send_conn.go	88.68% <88.89%> (+1.72%)	⬆️
sys_conn_helper_linux.go	76.12% <83.33%> (+0.71%)	⬆️
sys_conn_helper_nonlinux.go	20.00% <0.00%> (-5.00%)	⬇️

... and 1 file with indirect coverage changes