Add DKMS package on Debian-based distributions

* config/deb.am: Enable building DKMS packages for Debian
* rpm/generic/spl-dkms.spec.in: Adjust spec to be Debian-compatible
  * Condition kernel-devel Requires to RPM distros
  * Ensure that --rpm_safe_upgrade isn't used on non-RPM distros
* config/deb.am: Drop CONFIG_KERNEL and CONFIG_USER guards
* Makefile.am: Add pkg-dkms target

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Neal Gompa <ngompa@datto.com>
Closes #657