DOS: revised builtin termcap definitions and enable NANSI.SYS extensions dynamically

* The clrtobot capability (cd) is only used after goto(1,1), so it's safe to
  clear the entire screen.
* Consistently use ESC[0m to disable attributes.
  AFAIK, it's not possible to disable individual attributes only.
* al, AL, dl, DL, ic and dc are actually available, but only under NANSI.SYS.
  We now detect recent versions of NANSI.SYS dynamically and enable these extensions,
  which will speed up screen scrolling.
* Scrolling is now fixed under Dosbox.
  Obviously they don't support the NANSI.SYS extensions.
* You can still define %TERMCAP% even on DOS to load a customized termcap definitions
  file.

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