UDP: Introduce an experimental (undocumented for now) public API for …

[gdamore]

…UDP.

This exposes the UDP methods as nng_ methods, and adds support for Multicast Membership, which is useful in a variety of situations.

No documentation is provided, and applications should consider thios API experimental.

fixes #

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[codecov[bot]]

Codecov Report

Attention: Patch coverage is 57.62712% with 25 lines in your changes are missing coverage. Please review.

Project coverage is 79.54%. Comparing base (890d489) to head (56fce7d).

Files	Patch %	Lines
src/platform/posix/posix_udp.c	44.44%	25 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1838 +/- ## ========================================== + Coverage 79.43% 79.54% +0.11% ========================================== Files 95 95 Lines 21484 21543 +59 ========================================== + Hits 17066 17137 +71 + Misses 4418 4406 -12 ```

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