the last of the low-level input overhaul: make the global shortcut list

the last of the low-level input overhaul: make the global shortcut list
use NANO_NO_KEY to represent no key value, add the NANO_HISTORY_KEY
sentinel value, add support for many more escape sequences for those
terminals that generate them, add DB's extensibility tweaks to
do_yesno(), make ASCII 128 act like Backspace when -d is used, add
various cleanups to the low-level input functions, and make the last of
the low-level input routines use the new low-level input functions

git-svn-id: svn://svn.savannah.gnu.org/nano/trunk/nano@1599 35c25a1d-7b9e-4130-9fde-d3aeb78583b8