Fix DKMS package upgrade and packager

Running 'yum upgrade zfs-dkms' package could appear to work properly
and still leave you with no zfs modules installed.  This will occur
when only the zfs release, and not the version, are incremented.
This may be the case for a fast moving zfs-testing repository.

During the upgrade process DKMS will realize that zfs-x.y.z is already
installed and remove it.  DKMS then correctly builds the new modules
for zfs-x.y.z.  However, as a final step when the old zfs-x.y.z-r is
removed the %preun script runs and removes the newly build modules.
To handle this case the %preun script has been updated to only run
when the last version of zfs-dkms is removed.

This change also updated ChangeLog section based on the DKMS
reference spec file.

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