Add basic Xinerama support via new lib/DtXinerama

Add basic Xinerama support via new lib/DtXinerama

This adds a basic library and support to dtsession and dtlogin to
support Xinerama/Twinview, where multimple monitors are used to make
up an X11 screen.

The main goal here is to draw dialogs and such centered on a monitor,
rather than spread out over multiple monitors.

Might need to add sorting - as on my test system, what I would
consider monitor 0, appears to actually be monitor 1.  So a sort might
need to be added to sort the screens according to increasing x and y
offsets so it make sense to a user.

Also, this library is built statically and not documented.  Maybe it
could be 'filled' out and refactored/redesigned in the futre if need
be and suppoerted.

It is enabled via a define, CDE_USEXINERAMA in site.def.  It's a very
simple lib, so I do not expect any issues with the BSD's - it should
build and work fine, assuming your X server has the XINERAMA
extension, which I think pretty much all of them do at this point.