Convert more code to use new backend. Add some more sentinel strings

Convert more code to use new backend.  Add some more sentinel strings
for enter and backspace keys in case their control key alternates get
rebound.

git-svn-id: svn://svn.savannah.gnu.org/nano/trunk/nano@4223 35c25a1d-7b9e-4130-9fde-d3aeb78583b8