7277 zdb should be able to print zfs_dbgmsg's

illumos/illumos-gate@29bdd2f916366ece37c4748bca6b3d61f57a223b
https://github.com/illumos/illumos-gate/commit/29bdd2f916366ece37c4748bca6b3d61f57a223b

https://www.illumos.org/issues/7277

ztest always prints the debug messages (zfs_dbgmsg()) by calling
zfs_dbgmsg_print(). We should add a flag to zdb to make it do this as well
before exiting.

Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Igor Kozhukhov <ikozhukhov@gmail.com>
Approved by: Dan McDonald <danmcd@omniti.com>
Author: Pavel Zakharov <pavel.zakharov@delphix.com>