MFC r331707: MFV r331706:
9235 rename zpool_rewind_policy_t to zpool_load_policy_t

illumos/illumos-gate@5dafeea3ebd2dd77affc802bcb90f63faf01589f

We want to be able to pass various settings during import/open of a pool,
which are not only related to rewind. Instead of adding a new policy and
duplicate a bunch of code, we should just rename rewind_policy to a more
generic term like load_policy.

For instance, we'd like to set spa->spa_import_flags from the nvlist,
rather from a flags parameter passed to spa_import as in some cases we want
those flags not only for the import case, but also for the open case. One
such flag could be ZFS_IMPORT_MISSING_LOG (as used in zdb) which would
allow zfs to open a pool when logs are missing.

Reviewed by: Matt Ahrens <matt@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Pavel Zakharov <pavel.zakharov@delphix.com>