* Implemented support for LPT device, which sends output to the
  currently-installed NextZXOS printer driver ("P")

* Implemented support for XDEV device, which performs i/o using the
  currently-installed NextZXOS auxiliary driver ("X")

* Implemented additional Z-19 terminal features, allowing some graphical
  Heathkit games to run:
        ESC F          enter graphics mode (affects ASCII 94..126)
        ESC G          leave graphics mode
        ESC n          send cursor position report to input (in form ESC Y l c)
        ESC x 1        enable 25th-line support (only reachable by ESC Y l c)
        ESC x 5        suppress cursor
        ESC y 1        disable 25th-line support
        ESC y 5        enable cursor

* Implemented "Set BIOS features" API (C=4) in USERF function
  [experimental].