Fix vt100 escape sequence for showing and hiding the cursor in syscons.
It should toggle between 2 states, but it used a cut-down version of
support for a related 3-state syscons escape sequence and inherited
bugs from that. The usual misbehaviour was that hiding and showing
the cursor reset it to a global default.

Support for the 3-state sequence remains broken by aliasing to the 2-state
sequence. This works better but incompatibly for the 2 cases that it
supports.