Add xterm emulation

[davidrg]

terminfo usually (never?) includes an entry for Kermit 95 or C-Kermit for Windows. This makes it difficult to report the full capabilities of the CKW terminal emulator. We're limited to masquerading as some other often less capable terminal/terminal emulator that does have a terminfo entry. At the moment that mostly means claiming to be the Linux console terminal (set term type linux), but that doesn't cover all the features CKW supports or might like to support.

Of the terminals terminfo usually knows about, xterm is the most capable. Being able to claim to be xterm might result in better use of CKWs feature set.

In order to claim xterm emulation though, we'd have to actually implement most/all the various extra escape sequences xterm implements. This means implementing implementing a good chunk of the following (ReGIS and Tektronix probably aren't too necessary):

• Mouse reporting #85
• Add support for bracketed pastes #171
• 24bit colour support #220
• Add 256 colour support #194
• Sixel support #88
• ReGIS support #199
• Tektronix 4014 emulation #200
• DECLRMM and DECSLRM - set left/right margins #232
• Alternate screen buffer #231
• DEC Locator mode
• crossed-out character attribute
• double-underlined attribute

And probably many more.