revert some attempts at simplifying the search/replace history code that

revert some attempts at simplifying the search/replace history code that
were causing breakage; make "previous history" and "next history" use
separate keys; and display them as "^P" and "^N" instead of using one
sentinel value for both, so that clicking on them with the mouse works
again

git-svn-id: svn://svn.savannah.gnu.org/nano/trunk/nano@2497 35c25a1d-7b9e-4130-9fde-d3aeb78583b8