MFV r316862: 6410 teach zdb to perform object lookups by path

MFV r316862: 6410 teach zdb to perform object lookups by path

illumos/illumos-gate@ed61ec1da9132e570b0853386d0f78a32f852cd2
https://github.com/illumos/illumos-gate/commit/ed61ec1da9132e570b0853386d0f78a32f852cd2

FreeBSD note: this commit does not update the manual page.
The original change includes conversion of the manual page from *roff format
to mandoc format.  So, it is hard to extract the content change from
that.  I am going to replace our zdb manual page, which is an earlier
independent conversion, with a slighly modified version of the upstream page.

https://www.illumos.org/issues/6410
  This is primarily intended to ease debugging & testing ZFS when one is only
  interested in things like the on-disk location of a specific object's blocks,
  but doesn't know their object id. This allows doing things like the following
  (FreeBSD-based example):
          # zpool create -f foo da0
          # dd if=/dev/zero of=/foo/1 bs=1M count=4 >/dev/null 2>&1
          # zpool export foo
          # zdb -vvvvv -o "ZFS plain file" foo /1
          Object  lvl   iblk   dblk  dsize  lsize   %full  type
               8    2    16K   128K  3.99M     4M  100.00  ZFS plain file (K=inherit) (Z=inherit)
                                              168   bonus  System attributes
          dnode flags: USED_BYTES USERUSED_ACCOUNTED
          dnode maxblkid: 31
          path    /1
          uid     0
          gid     0
          atime   Thu Apr 23 22:45:32 2015
          mtime   Thu Apr 23 22:45:32 2015
          ctime   Thu Apr 23 22:45:32 2015
          crtime  Thu Apr 23 22:45:32 2015
          gen     7
          mode    100644
          size    4194304
          parent  4
          links   1
          pflags  40800000004
          Indirect blocks:
                0 L1  DVA[0]=<0:c19200:600> DVA[1]=<0:10800019200:600> [L1 ZFS
  plain file] fletcher4 lz4 LE contiguous unique double size=4000L/200P birth=7L/

Reviewed by: Pavel Zakharov <pavel.zakharov@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Will Andrews <will@freebsd.org>
Approved by: Dan McDonald <danmcd@omniti.com>
Author: Yuri Pankov <yuri.pankov@nexenta.com>

MFC after:	3 weeks