with -g one can open a new session - grouped with an existing session

That is, if DEBUG is set, show all tmux commands run to create a session

Only redo the layout if creating a new pane errors out due to missing
space. In HUGE multisession mode (on my machine, more than 200 panes in
one window), this gets starttime down to 25% of its previous value.

It also allows to be set per config file