First version of libdevq

This version contains three functions required by Mesa:
    1. get the absolute path in /dev of an open device
    2. get the PCI ID of an open device
    3. get the DRM kernel driver name of an open device

One current caveat is that the function returning the PCI ID is specific
to DRM devices.

Of course, this is a first shot and the API is subject to change.
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