print_vline can handle a NULL .line, so I can avoid the long comment and the ugly cast.

in part for symmetry, and in part because it seems a good spot to add an indicator for the presence of a client certificate in the future.

It's been changed recently (see history & killed files) and I don't want to keep documenting it: users shouldn't mess with their history file that easily (this is not an excuse for sloppy parsing, which isn't the case.) Furthermore, the format is easy, I think intuitive, and the code easily found in fs.c.

The other option, less intrusive, would be to send the killed tabs in reverse order, but it'd seems strange when looking at the session file otherwise. So, add a `append' flag to kill_tab to optionally append the killed tab instead of pre-pending it.