Preserve original fd blocking state in TLS I/O operations

[xbasel]

This change prevents unintended side effects on connection state and improves consistency with non-TLS sync operations.

[codecov]

Codecov Report

Attention: Patch coverage is 44.44444% with 5 lines in your changes missing coverage. Please review.

Project coverage is 70.66%. Comparing base (2df56d8) to head (a2e1b22).

Files with missing lines	Patch %	Lines
src/anet.c	44.44%	5 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable    #1298      +/-   ##
============================================
- Coverage     70.69%   70.66%   -0.03%     
============================================
  Files           114      114              
  Lines         63161    63169       +8     
============================================
- Hits          44650    44637      -13     
- Misses        18511    18532      +21     

Files with missing lines	Coverage Δ
src/tls.c	100.00% <ø> (ø)
src/anet.c	73.23% <44.44%> (-0.87%)	⬇️

... and 10 files with indirect coverage changes

[xbasel]

Please note that timeouts are still not restored. Not sure we should worry about timeouts.

[ranshid]

I agree that the timeout is not the thing we should keep track of, however I wonder why we cannot just flag the connection with indication that it is blocked instead of going through the fcntl all the time?