in do_replace_loop(), if the mark is on when we start, skip over all

in do_replace_loop(), if the mark is on when we start, skip over all
matches not found inside the marked text; this makes marked spell
checking when using the internal spell checker work as expected, and
also allows replacing only marked text when the mark is on

git-svn-id: svn://svn.savannah.gnu.org/nano/trunk/nano@1978 35c25a1d-7b9e-4130-9fde-d3aeb78583b8