Force IE to use the latest renderer available and disable compat mode

This was noted recently in [rt3 #21534], and RT really should have been
including it for a while now.

Compatibility mode can lead to bugs and lockups under some conditions
(see the CSS3Pie fiasco) and using an older IE rendering engine is never
better.
Merge branch '4.0/x-ua-compatible-header' into 4.0-trunk