Implement -t option to zpool import for temporary pool names

Originally, users had to handle spa namespace collisions by either
exporting the already imported pool or by specifying a new name for the
pool with a conflicting name. In the case of root pools from virtual
guests, neither approach to collision resolution is reasonable. This is
addessed by extending the new name syntax with a -t option to specify that
the new name is temporary. When specified, this sets an internal flag
that is passed into the kernel to tell it that all label updates should
refer to the name used in the original label. Consequently, the original
pool name will be retained on export.

Signed-off-by: Richard Yao <ryao@gentoo.org>