Two related changes to plan9port.

1. If you use rio and Firefox together, and you press F11
in Firefox to make Firefox go full-screen, rio gets out of the
way entirely now, so that Firefox really does get the whole
screen. Before, there was always a border on the top
and left sides. Firefox does not restore itself to the
correct location -- it ends up a border's width above
and to the left of its original position. If you want to find
this bug, it's all yours.

2. Typing F11 in a graphical Plan 9 window will resize the
window to take up the whole screen (I hesitate to call it
a separate "full-screen mode"); typing F11 again goes
back to the original size & location. I find it useful to
F11 acme to full screen to block out distractions.