Man page updates to go along with SVN r256343, introducing zfsboot.
Improve locking model used to protect netgraph topology:
use rwlocks instead of mutexes on node traversal.

Reviewed by:	glebius
Tested by:	Eugene Grosbein <egrosbein@rdtc.ru>
MFC after:	2 weeks
Sponsored by:	Yandex LLC
Bump .Dd date (should have been part of SVN r256549)
Unify periph invalidation and destruction reporting.
Print message containing device model and serial number on invalidation.

Requested by:   glebius
MFC after:	1 week
Make ZFSBOOT_BEROOT_NAME be ROOT by default. This is what sysutils/beadm
and OpenSolaris/Illumos beadm use.

Remove /usr/local and /var/db/pkg datasets.  Andriy Gapon writes:
I want to note that a good implementation of BEs should support a dependent
datasets feature.  Unfortunately, it seems that we do not have any good BE
implementation for FreeBSD right now.  If we had, personally I'd prefer to
have /usr/local in separate filesystem.

NOTE: Until then, remove these datasets.

Discussed on:	src-committers
Submitted by:	Bryan Drewery <bryan@shatow.net>
Reviewed by:	Allan Jude <freebsd@allanjude.com>
MFC after:	3 days