- Change add_undo and current_undo to only take action arg, as openfilestruct arg...

- Change add_undo and current_undo to only take action arg, as openfilestruct arg is always openfile
- Add ability to undo a file/cmd insert, needed to add undoable flag to read_file and open_buffer as a result

git-svn-id: svn://svn.savannah.gnu.org/nano/trunk/nano@4291 35c25a1d-7b9e-4130-9fde-d3aeb78583b8