[PATCH xserver] check for elevated privileges rather than just euid=0

[Antoine Martin]

This allows us to run the server as a normal user whilst still
being able to use the -modulepath, -logfile and -config switches
We define a xf86PrivsElevated which will do the checks and cache
the result in case it is called more than once.
Also renamed the paths #defines to match their new meaning.
Original discussion which led to this patch can be found here:
http://lists.freedesktop.org/archives/xorg-devel/2011-September/025853.html

Signed-off-by: antoine <antoine at nagafix.co.uk>
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