cleanup: Enforce stricter identifier naming using clang-tidy.

[iphydf]

This change is 

[codecov]

Codecov Report

Patch coverage: 90.78% and project coverage change: +0.08% 🎉

Comparison is base (a549807) 74.49% compared to head (4d3c97f) 74.57%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2404      +/-   ##
==========================================
+ Coverage   74.49%   74.57%   +0.08%     
==========================================
  Files          87       87              
  Lines       26194    26194              
==========================================
+ Hits        19512    19533      +21     
+ Misses       6682     6661      -21     

Files Changed	Coverage Δ
toxcore/TCP_connection.c	70.86% <71.42%> (ø)
toxcore/network.c	86.09% <84.61%> (ø)
toxcore/TCP_server.c	78.59% <86.95%> (-0.18%)	⬇️
toxcore/TCP_client.c	83.88% <97.05%> (+0.66%)	⬆️
toxcore/DHT.c	84.72% <100.00%> (+0.21%)	⬆️
toxcore/Messenger.c	82.67% <100.00%> (ø)
toxcore/TCP_common.c	78.72% <100.00%> (ø)
toxcore/friend_connection.c	85.77% <100.00%> (+0.22%)	⬆️
toxcore/group_chats.c	70.15% <100.00%> (+0.29%)	⬆️
toxcore/onion_client.c	90.31% <100.00%> (ø)

... and 4 files with indirect coverage changes

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