egl: Native Display autodetection

EGL doesnt define howto manage different native platforms.
So mesa has a builtime configurable default platform,
whith non-standard envvar (EGL_PLATFORM) overwrites.
This caused unneeded bugreports, when EGL_PLATFORM was forgotten.

Detection is grouped into basic types of NativeDisplays (which itself
needs to be detected).  The final decision is based on characteristcs
of these basic types:

  File Desciptor based platforms (fbdev):
    - fstat(2) to check for being a fd that belongs to a character device
    - check kernel subsystem (todo)

  Pointer to structuctures (x11, wayland, drm/gbm):
    - mincore(2) to check whether its valid pointer to some memory.
    - magic elements (e.g. pointers to exported symbols):
      o wayland display stores interface type pointer (first elm.)
      o gbm stores pointer to its constructor (first elm.)
      o x11 as a fallback (FIXME?)

Reviewed-by: Kristian Høgsberg <[email protected]>

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