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        These changes require rebuilding all.
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        * fhandler.h: Add mmap(), munmap() and msync() to fhandler_base
        and fhandler_disk_file.
        * mmem.cc (mmap): Eliminated device dependent implementation details.
        These are moved to the appropriate fhandler class.
        (munmap): Ditto.
        (msync): Ditto.
        (fhandler_base::mmap): New method.
        (fhandler_base::munmap): Ditto.
        (fhandler_base::msync): Ditto.
        (fhandler_disk_file::mmap): Ditto.
        (fhandler_disk_file::munmap): Ditto.
        (fhandler_disk_file::msync): Ditto.