Rename global variable "state" to "_cursesi_state".

Until now, if application happens to have a global variable of the same
name, it was overridden by curses routines. This is the scenario in
which aspell crashes when linked to our curses, reported in pkg/44005.

We need to wipe out global/static variables like "_cursesi_state" or
"wstate" for thread safety. But it would be a future task...

XXX pullup to netbsd-8 and netbsd-7

(rin)