add code to partition a filestruct between a set of arbitrary

add code to partition a filestruct between a set of arbitrary
coordinates; given the coordinates of the beginning and end of the mark,
this allows proper and easier handling of saving marked selections,
replacing text only in marked selections, and spell-checking marked
selections using either the internal or alternate spell checker; do all
these using a global partition structure

git-svn-id: svn://svn.savannah.gnu.org/nano/trunk/nano@2050 35c25a1d-7b9e-4130-9fde-d3aeb78583b8