Add DKMS package on Debian-based distributions

* config/deb.am: Enable building DKMS packages for Debian
* rpm/generic/zfs-dkms.spec.in: Adjust spec to be Debian-compatible
  * Condition kernel-devel Req to RPM distros
  * Adjust the DKMS Req to have a minimum of a version only
  * 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 #6044 
Closes #6731

rpm/generic/zfs-dkms.spec.in

@@ -1,5 +1,9 @@
 %{?!packager: %define packager Brian Behlendorf <behlendorf1@llnl.gov>}
 
+%if ! 0%{?rhel}%{?fedora}%{?mageia}%{?suse_version}
+%define not_rpm 1
+%endif
+
 %define module  @PACKAGE@
 %define mkconf  scripts/dkms.mkconf
 
@@ -16,10 +20,12 @@ Source0:        %{module}-%{version}.tar.gz
 BuildRoot:      %{_tmppath}/%{name}-%{version}-%{release}-root-%(%{__id_u} -n)
 BuildArch:      noarch
 
-Requires:       dkms >= 2.2.0.3-20
+Requires:       dkms >= 2.2.0.3
 Requires:       spl-dkms = %{version}
 Requires:       gcc, make, perl
+%if 0%{?rhel}%{?fedora}%{?mageia}%{?suse_version}
 Requires:       kernel-devel
+%endif
 Provides:       %{module}-kmod = %{version}
 
 %description
@@ -69,7 +75,7 @@ DKMS_META_ALIAS=`cat $CONFIG_H 2>/dev/null |
 if [ "$SPEC_META_ALIAS" = "$DKMS_META_ALIAS" ]; then
     echo -e
     echo -e "Uninstall of %{module} module ($SPEC_META_ALIAS) beginning:"
-    dkms remove -m %{module} -v %{version} --all --rpm_safe_upgrade
+    dkms remove -m %{module} -v %{version} --all %{!?not_rpm:--rpm_safe_upgrade}
 fi
 exit 0