Mostly from https://invisible-island.net/datafiles/current/termcap.src.gz:
- add "ecma+italics" cap
- use it in "xterm-basic"
- add "tmux"/"tmux-256color" caps, fixing "so" and "se" caps, and sourcing "ecma+italics" and "screen"/"screen-256color"
(Reviewers list based on commit history; please correct if I got it wrong.)