Replace NNG_OPT_SUB_SUBSCRIBE/UNSUBSCRIBE with functions.

[gdamore]

The main purpose is to eliminate the NNI_TYPE_OPAQUE options, by putting these into their own first class, protocol-specific, functions.

fixes #

Note that the above format should be used in your git commit comments.
You agree that by submitting a PR, you have read and agreed to our
contributing guidelines.

[codecov]

Codecov Report

Attention: Patch coverage is 98.27586% with 1 line in your changes missing coverage. Please review.

Project coverage is 80.02%. Comparing base (02ec0b5) to head (a5807e4).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/tools/perf/pubdrop.c	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1909      +/-   ##
==========================================
- Coverage   80.14%   80.02%   -0.12%     
==========================================
  Files          94       94              
  Lines       24232    24275      +43     
  Branches     3251     3259       +8     
==========================================
+ Hits        19420    19426       +6     
- Misses       4738     4777      +39     
+ Partials       74       72       -2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.