Add missing DATA_TYPE_STRING_ARRAY output

This functionality has always been missing. But until now there
were no zevents which included an array of strings so it wasn't
missed. However, that's now changed so to ensure this information
is output correctly by 'zpool events -v' the DATA_TYPE_STRING_ARRAY
has been implemented.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Chris Dunlap <cdunlap@llnl.gov>
Issue #2