Add option to zpool status to print guid

The use of vdev GUIDs are a necessary workaround in edge cases where the
names provided by `zpool status` are not accepted by the zpool
detach/offline/remove/replace commands make the use of vdev GUIDs
necessary. The current method of obtaining them uses zdb, but this does
not work in all cases (see zfsonlinux/zfs#1530).

This provides a method of obtaining vdev GUIDs that is more reliable and
straightforward than zdb. It would be better to fix all edge cases that
require the use of GUIDs as a workaround, but Linux's /dev design makes
it difficult to anticipate such edge cases.

Note that this adds a new boolean parameter to zpool_vdev_name, which
changes the libzfs interface.

Signed-off-by: Richard Yao <ryao@gentoo.org>