OpenZFS 6410 - teach zdb to perform object lookups by path

Authored by: Yuri Pankov <yuri.pankov@nexenta.com>
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>
Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>

Porting Notes:
- Replaced zdb.8 with upstream mdoc zdb.1m version.  Updated to
  include Linux specific features: -V verbatium imports and
  improved label printing (-u, and -l).
- Minor changes to `zdb -h` output to honor 80 character limit.

OpenZFS-issue: https://www.illumos.org/issues/6410
OpenZFS-commit: openzfs/openzfs@ed61ec1
Closes #6006