monitor: rename `screendump` command to `screenshot`

`screendump` is hard to memorize for occasional users. It is easier to remember `screenshot` name.

It would also be nice if monitor appended extension automatically if not set.

The monitor has TAB completion, so you should get the right command if you type "screen" + TAB. And since it's been quite a while since this ticket has been opened and nobody took care of this, I think it's quite unlikely that the name of the command gets changed, so closing this ticket now as "Won't fix" - sorry!