mincurses.c: use cursor_normal to disable cursor_invisible

There are actually three capabilities:

   cursor_invisible
   cursor_normal
   cursor_visible

The "cursor_normal" string disables both "cursor_invisible" and
"cursor_visible".  The actual meaning of "cursor_visible" is to
make the cursor "very visible."

Prefer using "cursor_normal" to disable "cursor_invisible".

Not all terminals have a "cursor_visible" string and it's not the
opposite of "cursor_invisible" anyway.

In the off chance that a terminal description has been created
incorrectly, if "cursor_normal" is not available but "cursor_visible"
is, it will be used instead (just like the previous code did).

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>
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