Allow 'zpool events' filtering by pool name

Allow 'zpool events' filtering by pool name

Additionally add four new tests:

 * zpool_events_clear: verify 'zpool events -c' functionality
 * zpool_events_cliargs: verify command line options and arguments
 * zpool_events_follow: verify 'zpool events -f'
 * zpool_events_poolname: verify events filtering by pool name

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #3285
Closes #6762