MFC r235212, r235216, r235217:

MFC r235212, r235216, r235217:

MFC r325212 [1]:
Import illumos changeset 13618:c6ae14a341e8
2088 zdb could use a reasonable manual page

Rewrite new zdb manpage to mdoc(7)

MFC r235216 [2]:
Add support for force unmounting ZFS filesystems during "zfs rename"
with the -f flag.

Reimplementation of the illumos changeset 13677:a0cbef703c12
2635 'zfs rename -f' to perform force unmount

MFC r235217 [3]:
Remove unreachable break in zfs_main.c

Partial import of illumos changeset 13622:e5889df1eaac
2077 lots of unreachable breaks in illumos gate

References:
https://www.illumos.org/issues/2088 [1]
https://www.illumos.org/issues/2635 [2]
https://www.illumos.org/issues/2077 [3]

PR:		164447 [1], 164447 [2], 167370 [3]
Suggested by:	Marcelo Araujo <araujo@FreeBSD.org>
Obtained from:	illumos (issue #2088 [1], #2635 [2], #2077 [3])

git-svn-id: svn://svn.freebsd.org/base/stable/8@235702 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f