http: fix mishandling of very long headers or URIs, and mishandling o…

[gdamore]

…f unicode

Also, nng_err is now a distinct type which might be nicer in debuggers.

fixes #

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.68%. Comparing base (c34abc2) to head (d88484c).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2084      +/-   ##
==========================================
+ Coverage   83.66%   83.68%   +0.02%     
==========================================
  Files          93       93              
  Lines       23503    23505       +2     
  Branches     3035     3035              
==========================================
+ Hits        19663    19671       +8     
+ Misses       3762     3754       -8     
- Partials       78       80       +2     

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