Added no_scrub_restart flag to zpool reopen

Added no_scrub_restart flag to zpool reopen

Added -n flag to zpool reopen that allows a running scrub
operation to continue if there is a device with Dirty Time Log.

By default if a component device has a DTL and zpool reopen
is executed all running scan operations will be restarted.

Added functional tests for `zpool reopen`

Tests covers following scenarios:
* `zpool reopen` without arguments,
* `zpool reopen` with pool name as argument,
* `zpool reopen` while scrubbing,
* `zpool reopen -n` while scrubbing,
* `zpool reopen -n` while resilvering,
* `zpool reopen` with bad arguments.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tom Caputi <tcaputi@datto.com>
Signed-off-by: Arkadiusz Bubała <arkadiusz.bubala@open-e.com>
Closes #6076
Closes #6746