refactor: Move some OS-specifics into tox_system.

[iphydf]

This change is 

[codecov]

Codecov Report

Attention: Patch coverage is 77.31844% with 203 lines in your changes are missing coverage. Please review.

Project coverage is 72.44%. Comparing base (102a1fa) to head (a3fc637).

❗ Current head a3fc637 differs from pull request most recent head 09f77d8. Consider uploading reports for the commit 09f77d8 to get more accurate results

Files	Patch %	Lines
toxcore/group_chats.c	74.24%	34 Missing ⚠️
toxcore/network_test_util.cc	0.00%	18 Missing ⚠️
toxcore/tox_system.c	0.00%	17 Missing ⚠️
testing/fuzzing/fuzz_support.cc	40.00%	12 Missing ⚠️
toxcore/os_network.c	79.31%	12 Missing ⚠️
toxcore/tox_network.c	52.00%	12 Missing ⚠️
toxcore/tox_log.c	0.00%	11 Missing ⚠️
toxcore/group_connection.c	83.33%	10 Missing ⚠️
toxcore/os_time.c	9.09%	10 Missing ⚠️
toxcore/tox.c	47.36%	10 Missing ⚠️
... and 17 more

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2397      +/-   ##
==========================================
- Coverage   73.08%   72.44%   -0.64%     
==========================================
  Files         149      162      +13     
  Lines       30531    30712     +181     
==========================================
- Hits        22313    22250      -63     
- Misses       8218     8462     +244     

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