Let `zpool import` ignore a missing hostid record.

Change the zpool program to skip its hostid mismatch check in the same way that
libzfs already does.

Invoked imports fail if the ZPOOL_CONFIG_HOSTID nvpair is missing in the
/etc/zfs/zpool.cache file, which can happen as of the /etc/hostid deprecation
in commit zfsonlinux/spl@acf0ade362cb8b26d67770114ee6fa17816e6b65.

Closes: #2794