NNG_OPT_RECVFD and NNG_OPT_SENDFD converted to functions.

[gdamore]

These options are removed entirely, and their functionality is now available via special functions, nng_socket_get_send_poll_fd and nng_socket_get_recv_poll_fd, making these first class methods on the socket.

This eliminates a bit of wasteful code, and provides type safety for these methods.

fixes #

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[codecov]

Codecov Report

Attention: Patch coverage is 96.96970% with 2 lines in your changes missing coverage. Please review.

Project coverage is 80.07%. Comparing base (9b27984) to head (279180c).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/nng.c	87.50%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1905      +/-   ##
==========================================
- Coverage   80.19%   80.07%   -0.12%     
==========================================
  Files          94       94              
  Lines       24260    24216      -44     
  Branches     3262     3246      -16     
==========================================
- Hits        19456    19392      -64     
- Misses       4731     4750      +19     
- Partials       73       74       +1     

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