MFV r303083: 7164 zdb should be able to open the root dataset

Note: conversion of the manual page change from roff to mdoc is mine.

illumos/illumos-gate@b702644a6eb66615d67b492fd73ecd9efa11fc7d
https://github.com/illumos/illumos-gate/commit/b702644a6eb66615d67b492fd73ecd9efa11fc7d

https://www.illumos.org/issues/7164

If the pool/dataset command-line argument is specified with a trailing
slash, for example, "tank/", we should interpret it as the topmost
dataset (rather than the whole pool)

Reviewed by: Pavel Zakharov <pavel.zakharov@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Tim Chase <tim@chase2k.com>
PR: 204661
MFC after: 1 week
Relnotes: yes