Don't do anything in the client callback if the client has already died to
avoid a use-after-free (the callback is used twice, once for the client itself
and once for the tty). Fixes crashes seen by Han Boetes.
Remove the -d flag to tmux and just use op/AX to detect default colours.
Irritatingly, although op can be used to tell if a terminal supports default
colours, it can't be used to set them because in some terminfo descriptions it
resets attributes as a side-effect (acts as sgr0) and in others it doesn't, so
it is not possible to determine reliably what the terminal state will be
afterwards. So if AX is missing and op is present, tmux just sends sgr0.
Anyone using -d for a terminal who finds they actually needed it can replace it
using terminal-overrides, but please let me know as it is probably an omission
from terminfo.
Split the server code handling clients, jobs and windows off into separate
files from server.c (merging server-msg.c into the client file) and rather than
iterating over each set after poll(), allow a callback to be specified when the
fd is added and just walk once over the returned pollfds calling each callback
where needed.
More to come, getting this in so it is tested.
