Move extensions from extmod to Xext/

[Tomas Carnecky]

This short series moves extensions which used to live in extmod into
the Xext directory. Some were easy to move (simply move the ExtensionModule
defintion), some require additional source files (XF86VidMode and DGA).
I moved them all into Xext so that hw/xfree86/dixmods/extmod can be removed
completely. XSELinux was a bit tricky in that it parses the config file
and looks for options in the Modules/extmod subsection.

The first patch is optional, just some leftover I found while going through
the code. The last patch technically also optional, but I think is a nice
cleanup (the term 'builtin' is too generic, better explicitely say
what the library is being linked into).