Ticket #3665: fix compatibility with netbsd curses.
The code that manipulates the ncurses backend into changing
the key combination to generate SIGINT from CTRL-c to CTRL-g does
so by accessing undocumented internal ncurses data structures.
This breaks compilation with netbsd-curses[0], and could also break
when the ncurses author decides to change internal structures in a
future release.
Fix it by using a portable approach that works everywhere using libc
primitives instead.
[0] https://github.com/sabotage-linux/netbsd-curses
Signed-off-by: Andrew Borodin <aborodin@vmail.ru>