Xresources: Enable Scroll Lock in xterm

Whenever I think xterm is missing a feature from other terminals it
seems I just have to read the man page one more time to find it, or
at least something close to it.

This one makes xterm's viewport keep showing the same set of lines even
as more output keeps coming by pressing the scroll lock key.

Ideally I'd like it to happen automatically if I scroll back like other
terminals can be configured to do, but this is close enough.