MFV r316864: 6392 zdb: introduce -V for verbatim import

MFV r316864: 6392 zdb: introduce -V for verbatim import

illumos/illumos-gate@dfd5965f7e43b6a630e5ac86708ae76b4f02cc40
https://github.com/illumos/illumos-gate/commit/dfd5965f7e43b6a630e5ac86708ae76b4f02cc40

FreeBSD note: the manual page is to be updated separately.

https://www.illumos.org/issues/6392
  When given a pool name via -e, zdb would attempt an import. If it
  failed, then it would attempt a verbatim import. This behavior is
  not always desirable so a -V switch is added to zdb to control the
  behavior. When specified, a verbatim import is done. Otherwise,
  the behavior is as it was previously, except no verbatim import
  is done on failure.
  https://github.com/zfsonlinux/zfs/commit/a5778ea2427bd340e3b4f697d9b6e1452bd71909
  Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
  Reviewed by: Matthew Ahrens <mahrens@delphix.com>

Reviewed by: Yuri Pankov <yuri.pankov@gmail.com>
Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed by: Matt Ahrens <mahrens@delphix.com>
Reviewed by: Pavel Zakharov <pavel.zakharov@delphix.com>
Approved by: Dan McDonald <danmcd@omniti.com>
Author: Richard Yao <ryao@gentoo.org>

MFC after:	3 weeks