Change spl-kmod-devel install path

Install the common spl kernel development headers under
/usr/src/spl-<version>/ rather than in a kernel specific
directory.  The kernel specific build products such as
spl_config.h and Modules.symvers are left installed under
/usr/src/spl-<version>/<kernel>.

This was done to be consistent with where dkms expects
kernel module source to be packaged.  It also allows for
a common spl-kmod-devel package which includes the headers,
and per-kernel spl-kmod-devel-<kernel> packages.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Refresh RPM packaging

Refresh the existing RPM packaging to conform to the 'Fedora
Packaging Guidelines'.  This includes adopting the kmods2
packaging standard which is used fod kmods distributed by
rpmfusion for Fedora/RHEL.

  http://fedoraproject.org/wiki/Packaging:Guidelines
  http://rpmfusion.org/Packaging/KernelModules/Kmods2

While the spec files have been entirely rewritten from a
user perspective the only major changes are:

* There is now a build dependency on kmodtool-1.22a.  This
  noarch package is available in the rpmfusion repositories
  and at zfsonlinux.org.

* The kernel binary module packages have been renamed from
  spl-modules-* to kmod-spl-* as specificed by kmods2.

* The is now a common kmod-spl-devel-* package in addition
  to the per-kernel devel packages.  The common package
  contains the development headers while the per-kernel
  package contains kernel specific build products.

* Currently there are only spec files for Fedora.  At the
  moment these spec files are used for all distributions and
  should be largely correct.  Infrastructure was added for
  distribution spec files which can be added later.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>