Implement -t option to zpool import for temporary pool names

Specifying newname to a pool import currently changes the pool name. We
implement option -t. Specifying option -t in conjuction with newpool to
zpool import will make the name change temporary. In specific, the
original pool name will be used in all label updates and therefore is
retained upon export.

This is useful in any situation where maintenance on a system's root
pool is done from another system. A good example is environments using
virtualization, where it is more desireable to do rollback of the root
dataset of a guest's root pool rather than rollback of the zvol hosting
the guest's root pool when an issue prevents the guest from booting
properly. Other examples include maintenance on root pools hosted on
iSCSI and operating system installations on USB keys that are used to
rescue non-bootable sytems.

Signed-off-by: Richard Yao <ryao@gentoo.org>