Closes: #4

Closes: #3

    tmux: /usr/lib/libncursesw.so.6: version `NCURSES6_TINFO_6.4.current' not
    found (required by tmux)

https://bbs.archlinux.org/viewtopic.php?pid=2172080

... as it turns out that tmux comes with its own implementation, and does
not depend in libsixel, at all. No idea what confused me... 

Follow-up on #1

Please give it some good testing...

This has been enabled for #1, but it pulls in quite a lot of dependencies.
These are not worth it installing on a clean server, so reverting.

Closes #1

Thank a lot to loqs for noting!

(The only change is the fix we had in 3.1_b-2.)

tmux has a stack overflow in CSI parsing.

OpenBSD 6.8 errata 003, October 29, 2020:

https://ftp.openbsd.org/pub/OpenBSD/patches/6.8/common/003_tmux.patch.sig