feat: Add self_disconnected exit type

[JFreegman]

This is mainly so that clients can prevent peer exit spam messages
when they disconnect from all peers at once

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This change is 

[ghost]

DeepCode's analysis on #15a7a4 found:

⚠️ 1 warning 1 minor issue.

👉 View analysis in DeepCode’s Dashboard

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[codecov]

Codecov Report

Merging #1606 into ngc will increase coverage by 0.11%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##              ngc    #1606      +/-   ##
==========================================
+ Coverage   81.34%   81.45%   +0.11%     
==========================================
  Files          95       95              
  Lines       20356    20356              
==========================================
+ Hits        16558    16581      +23     
+ Misses       3798     3775      -23     

Impacted Files	Coverage Δ
toxcore/group_chats.c	68.79% <50.00%> (-0.05%)	⬇️
toxcore/Messenger.c	86.24% <100.00%> (+0.22%)	⬆️
toxav/bwcontroller.c	42.10% <0.00%> (-7.90%)	⬇️
toxcore/TCP_connection.c	80.26% <0.00%> (-0.38%)	⬇️
toxcore/group.c	91.82% <0.00%> (-0.16%)	⬇️
toxcore/net_crypto.c	94.41% <0.00%> (+0.73%)	⬆️
toxcore/friend_connection.c	95.53% <0.00%> (+1.11%)	⬆️
toxcore/TCP_server.c	82.80% <0.00%> (+2.76%)	⬆️
... and 1 more

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