Basic xterm emulation

[davidrg]

This is an attempt at a usable emulation of xterm. In particular, a terminal type that passes most of the tests in esctest2 (https://github.com/ThomasDickey/esctest) and supports a reasonable subset of xterms extensions. The goal is maximum achievable compatibility with xterm, enough that using TERM=xterm doesn't cause any issues for a vast majority of applications.

Why? Because CKW has only emulation options. There is no "ckw" terminal type that turns on all available features, and even if there were there is the challenge of getting that terminal type into every terminfo database on every machine in the world. Faced with this same challenge a lot of other terminal emulators, eg PuTTY, just claim to be xterm. We're going to aim to do a better job of matching xterms behaviour though.

In order to be merged:

In order to be in a releasable state, the xterm emulation option really needs to support:

• A significant majority of esctest2 test cases
• The double-underlined character attribute
• The crossed-out character attribute
• DEC Locator mode
• Alternate screen buffer #231
• DECLRMM and DECSLRM - set left/right margins #232 - set left/right margins (VT420)
• DECRQCRA - request checksum of rectangular area (VT420), off by default and enabled with set term send-data on
• The same keymap as xterm
• Any xterm escape sequences we don't support should at least be stubbed out to ensure they do nothing rather than doing the wrong thing.

It would also be nice to support these extensions and other features, though perhaps not strictly necessary for a first version:

• Add support for bracketed pastes #171
• Add 256 colour support #194
• Sixel support #88

We also need to document what exactly the "xterm" terminal type is:

• What percentage of esctest2 we pass
• Which specific xterm features we do not implement and how CKW handles them. The list should be short enough that documenting it isn't too much of a problem.
• Any features the xterm emulation option supports that xterm does not implement (eg, PC Term mode)

We should also probably sort out some other emulation issues while we're here:

• Emulating xterm is going to bring in a significant number of VT420 features. Perhaps its worth finally adding a VT420 emulation option
• Might be worthwhile providing a "ckwterm" emulation type that just turns on as many features as possible. Maybe someday it makes its way into terminfo and we don't have to pretend to be xterm anymore.