Add add_prop_list_default helper

Adding to a property list only if there is no existing value is used
twice. Once by zpool create -R and again by zpool import -R. Now that
zpool create -t and zpool import -t also need it, lets refactor it into
a helper function to make the code more readable.

Signed-off-by: Richard Yao <ryao@gentoo.org>
zpool import -t should not update cachefile

zpool import's -t parameter is intended for use with -R when operating
on pools that belong to other systems. Like -R, pools imported in this
way should not update the cachefile unless explicitly requested. The
initial implementation allowed the cachefile to be updated when -R was
not used. This went uncaught during testing because -R had implicitly
disabled use of the cachefile.

Signed-off-by: Richard Yao <ryao@gentoo.org>
Implement -t option to zpool create for temporary pool names

Creating virtual machines that have their rootfs on ZFS on hosts that
have their rootfs on ZFS causes SPA namespace collisions when the
standard name rpool is used. The solution is either to give each guest
pool a name unique to the host, which is not always desireable, or boot
a VM environment containing an ISO image to install it, which is
cumbersome.

26b42f3f9d03f85cc7966dc2fe4dfe9216601b0e introduced `zpool import -t
...` to simplify situations where a host must access a guest's pool when
there is a SPA namespace conflict. We build upon that to introduce
`zpool import -t tname ...`. That allows us to create a pool whose
in-core name is tname, but whose on-disk name is the normal name
specified.

This simplifies the creation of machine images that use a rootfs on ZFS.
That benefits not only real world deployments, but also ZFSOnLinux
development by decreasing the time needed to perform rootfs on ZFS
experiments.

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