Do not load old CSS fixes for new browsers

Do not load old CSS fixes for new browsers

It's bad form to load a fix for "version X and later".  That means that
if the browser vendor fixes the underlying bug, they'll be served
incorrect markup and break.  On the other hand, if you check for just
"version X", then if they fix the bug they'll be fine, but if they
*don't* fix the bug they'll break, which is as it should be.  :)

Specifically, I disabled loading RTL fixes for Opera 9.6: my testing
shows that they worsen display, don't improve it.  All other fixes were
already not being loaded for browsers later than they should have been.
(Other than the five-year-old KHTML fix I removed outright in r53141,
which was loading for modern-day Safari and Chrome despite the
underlying bug very possibly having been fixed before WebKit even
existed . . .)

While I was at it, I made the variable names a bit saner.  I kept the
old weird ones (none are clear on which versions they include) for
backward compatibility in case scripts were relying on them.  Except
is_ff2_, which was so atrociously named that I had to put it out of its
misery.  For the others, I added new xxx_bugs variables to make it clear
that's all they were tracking.