Fix Ctrl-H mappings in Win32

Looks like pdcurses uses KEY_BACKSPACE for some other purpose than
backspace and Ctrl-H keys.  So <bs> mappings didn't work on Windows
ever and <c-h> mappings were broken same day they were fixed for *nix
by replacing L'\x0b' with the KEY_BACKSPACE macro.
Document fix of <c-h>/<bs> maps on Windows

Thanks to filterfalse.
Merge branch 'fix-ctrl-h-maps-on-win32'