Replace sysctl summary with tunables summary.

The original script displayed tunable parameters using sysctl calls.
This patch modifies this by displaying tunable parameters found in
/sys/modules/zfs/parameters/. modinfo calls are used to capture
descriptions.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Kyle Blatter <kyleblatter@llnl.gov>
Signed-off-by: Ned Bass <bass6@llnl.gov>