Only allow zfs.fs to autoimport pools with status ZPOOL_STATUS_OK.

This should prevent pools from being autoimported prematurely, in a
degraded state, before all devices have become available.

Removed the execvp version of zpool_import_by_guid() for now.

Only use zpool_read_cachefile() if ZFS_AUTOIMPORT_ZPOOL_CACHE_ONLY
is defined.