Fix 'zpool events' formatting for awk

Fix 'zpool events' formatting for awk

To make the 'zpool events' output simple to parse with awk the extra
newline after embedded nvlists has been dropped.  This allows the
entire event to be parsed as a single whitespace seperated record.

The -H option has been added to operate in scripted mode.  For the
'zpool events' command this means don't print the header.  The usage
of -H is consistent with scripted mode for other zpool commands.