Add space and ctrl-b keybindings to page down/up

[maoe]

They are the same key bindings as less.

[asayers]

Looking at the manpage, it looks like less accepts 'b' with or without a ctrl modifier. If we mimic that behaviour we can simplify the code a bit. I'll confirm that less actually behaves this way when I get home.

[asayers]

While we're at it, should we also add 'f' for pagedown?

[maoe]

Sure