overhaul the cutting and uncutting routines and the justify-related

overhaul the cutting and uncutting routines and the justify-related
routines to back up and restore unjustified text to use the partitioning
code, as it greatly simplifies how they work; also back up and restore
unjustified text in its own buffer instead of the cutbuffer, and add a
few minor bug fixes to the overhauled routines

git-svn-id: svn://svn.savannah.gnu.org/nano/trunk/nano@2124 35c25a1d-7b9e-4130-9fde-d3aeb78583b8