MFC r355974 (by rlibby):

libdevdctl: g++9 avoid Wdeprecated-copy

g++9 now warns about having defined an assignment operator but using the
default copy constructor, or vice versa. Avoid the issue in libdevdctl
by just using the default assignment operator too.

Reviewed by: asomers, dim
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D22887

MFC r325011, r325016

r325011:
zfsd should be able to online an L2ARC that disappears and returns

Previously, this didn't work because L2ARC devices' labels don't contain
pool GUIDs. Modify zfsd so that the pool GUID won't be required:

lib/libdevdctl/guid.h
Change INVALID_GUID from a uint64_t constant to a function that
returns an invalid Guid object. Remove the void constructor.
Nothing uses it, and it violates RAII.

cddl/usr.sbin/zfsd/case_file.h
cddl/usr.sbin/zfsd/case_file.cc
Allow CaseFile::Find to match a CaseFile based on Vdev GUID alone.
In CaseFile::ReEvaluate, attempt to online devices even if the newly
arrived device has no pool GUID.

cddl/usr.sbin/zfsd/vdev_iterator.cc
Iterate through a pool's cache devices as well as its regular
devices.

Reported by: avg
Reviewed by: avg
Sponsored by: Spectra Logic Corp
Differential Revision: https://reviews.freebsd.org/D12791

r325016:
Partially revert r325011: restore Guid's default constructor

Reported by: ohartmann
X-MFC-With: 325011
Sponsored by: Spectra Logic Corp

Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

zfsd(8), the ZFS fault management daemon

Add zfsd, which deals with hard drive faults in ZFS pools. It manages
hotspares and replements in drive slots that publish physical paths.

cddl/usr.sbin/zfsd
Add zfsd(8) and its unit tests

cddl/usr.sbin/Makefile
Add zfsd to the build

lib/libdevdctl
A C++ library that helps devd clients process events

lib/Makefile
share/mk/bsd.libnames.mk
share/mk/src.libnames.mk
Add libdevdctl to the build. It's a private library, unusable by
out-of-tree software.

etc/defaults/rc.conf
By default, set zfsd_enable to NO

etc/mtree/BSD.include.dist
Add a directory for libdevdctl's include files

etc/mtree/BSD.tests.dist
Add a directory for zfsd's unit tests

etc/mtree/BSD.var.dist
Add /var/db/zfsd/cases, where zfsd stores case files while it's shut
down.

etc/rc.d/Makefile
etc/rc.d/zfsd
Add zfsd's rc script

sys/cddl/contrib/opensolaris/uts/common/fs/zfs/vdev.c
Fix the resource.fs.zfs.statechange message. It had a number of
problems:

It was only being emitted on a transition to the HEALTHY state.
That made it impossible for zfsd to take actions based on drives
getting sicker.

It compared the new state to vdev_prevstate, which is the state that
the vdev had the last time it was opened. That doesn't make sense,
because a vdev can change state multiple times without being
reopened.

vdev_set_state contains logic that will change the device's new
state based on various conditions. However, the statechange event
was being posted _before_ that logic took effect. Now it's being
posted after.

Submitted by: gibbs, asomers, mav, allanjude
Reviewed by: mav, delphij
Relnotes: yes
Sponsored by: Spectra Logic Corp, iX Systems
Differential Revision: https://reviews.freebsd.org/D6564