Add support to UVM to allow processes to map pages directly from underlying objects that are already mapped into memory: for example, files in tmpfs; files in mfs; and also files on memory-mapped storage devices, like raw flash chips or experimental storage-class memory hardware.

This allows accesses (most notably, execution of user programs) to go directly to the backing object without copying the pages into main system memory. (Which in turn has obvious advantages.)

Note that uebayasi@ was working on this at one point but his changes did not get merged.

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