dev_ns: adopt the framebuffer subsystem

    Adds device namespace logic to the framebuffer subsystem.

    Isolate framebuffer state between device namespaces, and multiplex ac~
    to hardware state between the virtual framebuffers.

    Associate with each device namespace a shadow, virtual 'struct fb_inf~
    (vinfo) and a backing buffer, for each registered hardware framebuffe~
    Access from an active (foreground) device namespace is passed-through
    directly to the underlying device. Access from a passive (background)
    device namespace is directed to the virtal fb_info.

    The backing buffer is used for mmap-ed memory in passive namespace, so
    any drawing occurs to the backing buffer instead of to the real drive~
    When the active namespace changes, perform a context-switch and swap
    such mappings: remap memory previously in foreground to the backing
    buffer, and from background to the underlying device. The contents of
    the framebuffer is saved and restored in (from) the bacgkround (activ~
    backing buffer, respectively.

    Changelog:
    [16-Aug-13] v1 - initial version