function is an extension to the curses library.
It allows the caller to change the result of unctrl
and suppress related checks within the library that would normally
cause nonprinting characters to be rendered in visible form.
This affects only 8-bit characters.
The level parameter controls the result:
the library functions normally,
rendering nonprinting characters as described in unctrl.
the library ignores isprintf for codes in the range 160-255.
the library ignores isprintf for codes in the range 128-255.
It also modifies the output of unctrl, showing codes in the
range 128-159 as is.