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

Remove C constructs that are incompatible with C++ from various
OpenSolaris and ZFS header files. These changes are sufficient
to allow a C++ program to use the libzfs library.

Note: The majority of these files already included 'extern "C"'
declarations, so the intention of providing C++ compatibility
already existed even if it wasn't provided.

cddl/compat/opensolaris/include/assert.h:
Wrap our compatibility assert implementation in
'extern "C"'. Since this is a compatibility header
I matched the Solaris style of doing this explicitly
rather than rely on FreeBSD's __BEGIN/END_DECLS macro.

sys/cddl/compat/opensolaris/sys/kstat.h:
sys/cddl/contrib/opensolaris/uts/common/fs/zfs/sys/arc.h:
sys/cddl/contrib/opensolaris/uts/common/fs/zfs/sys/dsl_pool.h:
sys/cddl/contrib/opensolaris/uts/common/fs/zfs/sys/ddt.h:
sys/cddl/contrib/opensolaris/uts/common/fs/zfs/sys/spa.h:
sys/cddl/contrib/opensolaris/uts/common/fs/zfs/sys/zio.h:
Rename parameters in function declarations that conflict
with C++ keywords. This was the solution preferred by
members of the Illumos community.

sys/cddl/contrib/opensolaris/uts/common/fs/zfs/sys/zfs_ioctl.h:
In C, nested structures are visible in the global namespace,
but in C++, they take on the namespace of the structure in
which they are contained. Flatten nested structure
definitions within struct zfs_cmd so these structures are
visible in the global namespace when compiled in both
languages.

Sponsored by: Spectra Logic Corporation

Implement __assert() for Solaris-specific code. Until now Solaris code was
using Solaris prototype for __assert(), but FreeBSD's implementation.
Both take different arguments, so we were either core-dumping in assert()
or printing garbage.

Reported by: avg
MFC after: 1 week