Use tmux native terminfo instead of screen-256color

[prashantv]

macOS uses an old version of ncurses which doesn't contain a terminfo entry for tmux's native TERM: tmux-256color.

using screen-256color works reasonably well, but doesn't support italics, see https://github.com/tmux/tmux/wiki/FAQ#i-dont-see-italics-or-italics-and-reverse-are-the-wrong-way-round

This can be imported using the tic command,

$ /usr/bin/tic -x -o $HOME/.local/share/terminfo tmux-256color.src

and setting the TERMINFO_DIRS to look in that location,

export TERMINFO_DIRS="$TERMINFO_DIRS:$HOME/.local/share/terminfo"

Example src file:
https://gist.github.com/nicm/ea9cf3c93f22e0246ec858122d9abea1