tmux 2.6 now supports setting the 256 color palette

[Timothy Allen]

According to the tmux changelog:

https://github.com/tmux/tmux/blob/master/CHANGES#L312
https://github.com/tmux/tmux/blob/master/CHANGES#L115

...as of tmux 2.6, it now supports modifying the 256-color palette with
OSC 4/104. It would be good if you could update the terminfo database
to match - I think all that needs to happen is for the "tmux-256color"
entry to `use=xterm+256color` instead of `use=xterm+256setaf`.

Thanks!