* zellij: Add global `DEBUG_MODE` variable
that tells us whether zellij was started with the `--debug` CLI flag.
* utils/errors: Only log thread_bus message in debug
mode, and discard the message otherwise.
* utils/logging: Increase logsize to 16 MiB
per logfile, totaling 32 MiB of logs at most (in two files).
* zellij: Set global `DEBUG` variable in server
thread and make sure the value of the `--debug` CLI flag is propagated
to the server, too.
This means that to enable debug mode, the server must be started with
the `--debug` flag. This happens when the first client that starts the
zellij session has the `--debug` flag set, because it will be forwarded
to the server. Subsequent clients attaching to the same session with the
`--debug` flag specified **do not** override the value of the `DEBUG`
variable. Hence, if the server wasn't started in debug mode, this cannot
be changed.
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