Add extension terminfo(5) capabilities for margins.

Remove support for iTerm2's DSR 1337 extension and use the CSI > q
extension now supported by a few different terminals.

Move the background colour to clear with (if any) up as well as the data
when scrolling, redraw problem reported by sthen@.

256 and RGB features can imply AX (for aixterm colours).

Apply terminal-overrides after terminal detection, it always takes
precedence.

Change the Sync capability to be a string instead of a flag.

Tidy up the terminal detection and feature code and add named sets of
terminal features, each of which are defined in one place and map to a
builtin set of terminfo(5) capabilities. Features can be s

Tidy up the terminal detection and feature code and add named sets of
terminal features, each of which are defined in one place and map to a
builtin set of terminfo(5) capabilities. Features can be specified based
on TERM with a new terminal-features option or with the -T flag when
running tmux. tmux will also detect a few common terminals from the DA
and DSR responses.

This is intended to make it easier to configure tmux's use of
terminfo(5) even in the presence of outdated ncurses(3) or terminfo(5)
databases or for features which do not yet have a terminfo(5) entry.
Instead of having to grok terminfo(5) capability names and what they
should be set to in the terminal-overrides option, the user can
hopefully just give tmux a feature name and let it do the right thing.

The terminal-overrides option remains both for backwards compatibility
and to allow tweaks of individual capabilities.

tmux already did much of this already, this makes it tidier and simpler
to configure.

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