Check if key file was opened correctly in DHT_bootstrap.c

Warn the user if it wasn't.
Joining a group chat should be more reliable.

Only disconnect from the friend we used to join when we are connected
to at least 3 other people.
Group chats should be less likely to break.
Prevent joining groupchat more than once.
Removed useless code.
Groupchat stability fixes.

Query for peers if we were connected to nobody for some reason.
Comment fix.