cleanup: Add more const where possible.

[iphydf]

This change is 

[codecov]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (511bfe3) 73.67% compared to head (f70e588) 73.71%.

Files	Patch %	Lines
toxcore/Messenger.c	50.00%	2 Missing ⚠️
toxcore/DHT.c	66.66%	1 Missing ⚠️
toxcore/TCP_client.c	66.66%	1 Missing ⚠️
toxcore/TCP_connection.c	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2622      +/-   ##
==========================================
+ Coverage   73.67%   73.71%   +0.03%     
==========================================
  Files         148      148              
  Lines       30407    30407              
==========================================
+ Hits        22401    22413      +12     
+ Misses       8006     7994      -12     

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