Add zfs_autoimport_disable tunable

There are times when it is desirable for zfs to not automatically
populate the spa namespace at module load time using the pools
in the /etc/zfs/zpool.cache file. The zfs_autoimport_disable
module option has been added to control this behavior.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #330