Introduce the API for device query and enumeration. Those at the moment
produce nothing useful since zero devices are actually available.

That contradicts with the spec, so the extension isn't advertised just
yet.

With later commits we'll add support for software (always) and hardware
devices. Each one exposing the respective extension string.

v2:
 - fold API boilerplate into this patch
 - move _eglAddDevice, _eglDeviceSupports, _eglRefreshDeviceList to this
patch (Eric, Mathias)
 - make _eglFiniDevice the one called last

v3:
 - comment on the dummy _egl_device_extension enum entry (Eric)
 - annotate dev as MAYBE_UNUSED (Mathias)

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Mathias Fröhlich <Mathias.Froehlich@web.de>