Adds `server_log()` method in order to export the `print()` calls.
Centralize _log_, _broadcast_ & _send_ operations in the bottom of the
loop (except when sockets needs to be close).
I think it is improves readability and will facilitate code evolution,
for example when writing the servers log in file(s).
Now sends messages only to named-client, and for that uses
`list_sockets()`
Takes sender name from `_user_name` attribute and delete the name
argument from the method
`list_sockets()` uses only clients with a name already typed in
(filled in `_user_name` attribute), to avoid starting a game with not
named player
`count_clients()` was too messy and autonomous, now it uses
`list_sockets()` for counting only named players and not depend on
naming player mechanism or main socket.# Veuillez saisir le message de
validation pour vos modifications. Les lignes
