allow winrestview({'curswant':5})

Following a discussion from
https://groups.google.com/d/msg/vim_dev/xTa0kHWkY_o/ZXDq-HCifWsJ
https://groups.google.com/forum/#!topic/vim_dev/kydw6-vTuvM
https://groups.google.com/d/msg/vim_dev/o9GRXaJMwHg/9g8INNnNB9YJ
allow to call winrestview with a dictionary that does not contain
all values returned by winsaveview() so that it is possible to only
restore certain values as suggested by ZyX.