OK, this patch adds support for making the buffer strongly dedicated
with a prefix argument, and also adds an indicator to the mode line of
whether the current window is dedicated (which seems useful even if we
don't apply the toggle part of this patch).

I wonder if we should support clicking on the mode line indicator to
turn off dedicated status?  The tricky thing is that the mode line
indicator disappears when the window isn't dedicated, so there's no way
to turn it back *on* using the mouse.