x11-servers/xorg-server: Disable AIGLX by default

* Many of the existing 3D drivers in graphics/libGL are not built due
  to missing gallium support

* xorg-server 1.12 crashes when trying to load nonexistent 3D drivers

* Avoid the crashes in the default configuration by disabling AIGLX support

* This setting can be overriden by adding  Option "AIGLX"  "TRUE"
  in the ServerFlags section of xorg.conf