[BUG/TEST] Refaxed and fixed multiplexer reusage. Fixed ReuseAddr tests.

[ethouris]

Fixes #2389

Changes:

1. Reworked the procedure of finding the matching or conflicting multiplexer (some IPv6-related cases were incorrectly checking the cases of IPv6 only and IPv6+IPv4).
2. Obtaining the IPPROTO_IPV6/IPV6_V6ONLY flag from the socket in case of AF_INET6 family just after socket creation so that this flag can be checked when searching for a matching multiplexer binding.
3. Reworked and fixed UT for ReuseAddr.* - removed or put under a better control tests that were relying on a half-cleanedup previously bound sockets.

[codecov-commenter]

Codecov Report

Merging #2608 (d777686) into master (30f6f6b) will decrease coverage by 0.51%.
The diff coverage is 52.02%.

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@@            Coverage Diff             @@
##           master    #2608      +/-   ##
==========================================
- Coverage   66.53%   66.03%   -0.51%     
==========================================
  Files          99       99              
  Lines       19843    20043     +200     
==========================================
+ Hits        13203    13235      +32     
- Misses       6640     6808     +168     

Impacted Files	Coverage Δ
srtcore/api.h	96.55% <ø> (ø)
srtcore/channel.cpp	43.42% <0.00%> (-2.21%)	⬇️
srtcore/channel.h	0.00% <0.00%> (ø)
srtcore/api.cpp	52.72% <41.53%> (-0.48%)	⬇️
test/test_reuseaddr.cpp	64.85% <58.76%> (-15.44%)	⬇️
srtcore/socketconfig.h	90.00% <100.00%> (+0.16%)	⬆️
srtcore/list.cpp	76.69% <0.00%> (-9.28%)	⬇️
srtcore/window.h	96.72% <0.00%> (-3.28%)	⬇️
haicrypt/hcrypt_tx.c	17.14% <0.00%> (-1.28%)	⬇️
srtcore/core.cpp	60.92% <0.00%> (-0.33%)	⬇️
... and 5 more

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[stevomatthews]

Review pass completed with minor edits/suggestions.