Update build system and packaging

Minimal changes required to integrate the SPL sources in to the
ZFS repository build infrastructure and packaging.

Build system and packaging:

• Renamed SPL_* autoconf m4 macros to ZFS_*.
• Removed redundant SPL_* autoconf m4 macros.
• Updated the RPM spec files to remove SPL package dependency.
• The zfs package obsoletes the spl package, and the zfs-kmod package obsoletes the spl-kmod package.
• The zfs-kmod-devel* packages were updated to add compatibility symlinks under /usr/src/spl-x.y.z until all dependent packages can be updated. They will be removed in a future release.
• Updated copy-builtin script for in-kernel builds.
• Updated DKMS package to include the spl.ko.
• Updated stale AUTHORS file to include all contributors.
• Updated stale COPYRIGHT and included the SPL as an exception.
• Renamed README.markdown to README.md
• Renamed OPENSOLARIS.LICENSE to LICENSE.
• Renamed DISCLAIMER to NOTICE.

Required code changes:

• Removed redundant HAVE_SPL macro.
• Removed _BOOT from nvpairs since it doesn't apply for Linux.
• Initial header cleanup (removal of empty headers, refactoring).
• Remove SPL repository clone/build from zimport.sh.
• Use of DEFINE_RATELIMIT_STATE and DEFINE_SPINLOCK removed due to build issues when forcing C99 compilation.
• Replaced legacy ACCESS_ONCE with READ_ONCE.
• Include needed headers for current and EXPORT_SYMBOL.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Olaf Faaland <faaland1@llnl.gov>
Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Pavel Zakharov <pavel.zakharov@delphix.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
TEST_ZIMPORT_SKIP="yes"
Closes #7556