Rework the changes to DTRACE/ZFS files that were fixing Clang/LLVM build

Rever changes to upstream files and replace them with additional -Wno-
compiler argument flags.

Tested with base LLVM.

Requested by <chuq>

branches: 1.2.2;
Correct compilation of osnet/dev/dtrace/x86/dis_tables.c under Clang

Constify dgr_suffix.

This could be done with -W flags, but they are incompatible between
compilers.

This is a step forward functional MKLLVM=yes HAVE_LLVM=yes build.

Sponsored by <The NetBSD Foundation>

add some new files missed by my script:
merge a new version of the CDDL dtrace and ZFS code.
this changes the upstream vendor from OpenSolaris to FreeBSD,
and this version is based on FreeBSD svn r315983.

in addition to the 10 years of improvements from upstream,
this version also has these NetBSD-specific enhancements:
- dtrace FBT probes can now be placed in kernel modules.
- ZFS now supports mmap().