split at root broken when statusbar is full screen

If in cfg_statusbar.lua I set fullsize=true, split at root stops working.
Tested in ion-3rc-20070506.

A lesser problem is that a root split doesn't do anything when there's only
one frame.

A behavior that you may or may not consider a problem is the way such splits
are merged. If I split the screen twice normally, a frame always merges with
a frame that is adjacent to it. If I split the screen twice, but the second
time is a root split, the right/bottom frame merges with the left/top one
instead of with the middle one.

~fantasai