gallium: Add pipe loader for device enumeration and driver multiplexing.

The goal is to have a uniform interface to create winsys and
pipe_screen instances for any driver, exposing the device enumeration
capabilities that might be supported by the operating system (for now
there's a "drm" back-end using udev and a "sw" back-end that always
returns the same built-in devices).

The typical use case of this library will be:
>
> struct pipe_loader_device devs[n];
> struct pipe_screen *screen;
>
> pipe_loader_probe(&devs, n);
>[pick some device from the array...]
>
> screen = pipe_loader_create_screen(dev, library_search_path);
>[do something with screen...]
>
> screen->destroy(screen);
> pipe_loader_release(&devs, N);
>

A part of the code was taken from targets/gbm/pipe_loader.c, which
will be removed and replaced with calls into this library by a future
commit.

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