OpenZFS 6551 - cmd/zpool: cleanup gcc warnings

Porting Notes:

• Many of the fixes proposed by this patch were already applied.

In the cases where a different but equivalent fix was made the
code was updated with the OpenZFS version to minimize differences.

• The zpool_get_vdev_by_name() function was previously removed

by commit 235db0a.

Authored by: Igor Kozhukhov <ikozhukhov@gmail.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Andy Stormont <astormont@racktopsystems.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Haakan T Johansson <f96hajo@chalmers.se>
Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>

OpenZFS-issue: https://www.illumos.org/issues/6551
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/b327cd3
Closes #5590