termcap: add italics and tmux entries

• 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"

Reviewed by: bapt
Differential revision: https://reviews.freebsd.org/D18363