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

man/man8/zpool.8

@@ -25,6 +25,7 @@
 .\" Copyright (c) 2017 Datto Inc.
 .\" Copyright (c) 2017 George Melikov. All Rights Reserved.
 .\" Copyright 2017 Nexenta Systems, Inc.
+.\" Copyright (c) 2017 Open-E, Inc. All Rights Reserved.
 .\"
 .Dd August 23, 2017
 .Dt ZPOOL 8 SMM
@@ -140,6 +141,7 @@
 .Ar pool
 .Nm
 .Cm reopen
+.Op Fl n
 .Ar pool
 .Nm
 .Cm remove
@@ -1739,9 +1741,14 @@ performing this action.
 .It Xo
 .Nm
 .Cm reopen
+.Op Fl n
 .Ar pool
 .Xc
 Reopen all the vdevs associated with the pool.
+.Bl -tag -width Ds
+.It Fl n
+Do not restart an in-progress scrub operation. This is not recommended and can
+result in partially resilvered devices unless a second scrub is performed.
 .It Xo
 .Nm
 .Cm remove