I am trying to figure out how to create a new function that will scroll
up a screen if possible, or go the beginning of the buffer otherwise; I
would then bind this to M-v.  Ultimately, I want to have the corollary
behavior for C-v.

I can't figure out how to test whether it is possible to scroll up/down
an entire screen.  I found (goto-char (point-min/max)) , so that part
should be okay.

Matthew