Import illumos changeset 13618:c6ae14a341e8 [1]:
2088 zdb could use a reasonable manual page

Rewrite new zdb manpage to mdoc(7)

References:
https://www.illumos.org/issues/2088

PR: kern/167467 [1]
Submitted by: Marcelo Araujo <araujo@FreeBSD.org> [1]
Reviewed by: mm
Obtained from: Illumos (issue #2088)
MFC after: 1 week