New keybindings code and backend. New structs subnfunc for menu functions

New keybindings code and backend. New structs subnfunc for menu functions
and toggles and sc for shortcut keys, old 'shortcut' and 'toggles' structs are
gone.  The current implementation has a bunch of broken stuff (some of which is
documented in BUGS).  Updated nanorc.5 with some mostly complete documentation
on configuring.

git-svn-id: svn://svn.savannah.gnu.org/nano/trunk/nano@4215 35c25a1d-7b9e-4130-9fde-d3aeb78583b8