Add support for XTGETTCAP

How it works is available at https://invisible-island.net/xterm/ctlseqs/ctlseqs.html .

This is used to query for terminfo features and not relying on the actual files. It's supported by notcursors, most modern terms, etc. It could be sort of good when you ssh into system without TERM info you have and it could help terminals not having their terminfo spread to work with at least modern toolkits.