Correct reference counting of cached text segment images.  Image.ref should
always be equal to the number of pointers from Segment and Page structures
to the Image.  Without this patch, the kernel will leak Images, as shown
by this one-liner which will kill any Plan 9 system (change 200 to 2000
for a cpu server):
% for (i in `{seq 1 200}) {cp /bin/echo /tmp; /tmp/echo -n & /tmpe/echo $i}