[issue1863] Implement 'hammer volume-list' subcommand

[Matthew Dillon]

    ioctls copy data into/out of kernel space for you, but the
    size of the structure is built into the ioctl command so it
    cannot change without changing the ioctl command.  It's also
    fairly limited.  So anything that is potentially large needs
    to be passed in as a separate user pointer embedded in
    the ioctl command structure.

					-Matt
					Matthew Dillon 
					<dillon at backplane.com>