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

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
MFC after:	3 weeks
Sponsored by:	Spectra Logic Corp
Differential Revision:	https://reviews.freebsd.org/D12791