Use TERM=xterm-256color (or better) when truecolor is supported.

The current situation is that mosh will manually test for color support using terminfo, which makes sense, but then it only checks to see if the value is 256 or not. If it's 256, it sets xterm-256color and if not, it sets xterm:

https://github.com/mobile-shell/mosh/blob/1105d481bb9143dad43adf768f58da7b029fd39c/src/frontend/mosh-server.cc#L572-L578

This is a problem when terminals support truecolor and correctly advertise that they support 16M colors via terminfo, e.g.

$ tput -T alacritty-direct colors
16777216

In this scenario, mosh says, "Well.. it's not 256 so I guess it's 8," and then sets TERM=xterm. Then everything that is run inside the session, including tmux, thinks that not even 256 colors are supported, let alone 16M colors. It would be great if COLORTERM=truecolor were sufficient, but unfortunately it's not. There are too many applications that only look at $TERM (via terminfo) and are thus broken :(.

Start at src/frontend/mosh-server.cc lines 572-578 and compare its terminfo color-count handling with the truecolor example using tput -T alacritty-direct colors. Done means terminals advertising truecolor no longer fall back to TERM=xterm, while existing 256-color behavior remains correct.