Squashed commit of the following:

    fs/mount:  Implements procfs /proc/fs/blocks and /proc/fs/usage files, replacing the NSH df command.
    fs/mount:  Implements procfs /proc/fs/mount file, replacing the NSH mount command when there are not arguments.
    fs/: Move prototype of foreach_mountpoint out of include/nuttx/fs/fs.h to fs/mount/mount.h.  Add framework for the mount procfs (initial commit is just a close of the net/route table procfs.