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

config/deb.am

@@ -15,17 +15,23 @@ deb-local:
 	fi)
 
 deb-kmod: deb-local rpm-kmod
-if CONFIG_KERNEL
 	name=${PACKAGE}; \
 	version=${VERSION}-${RELEASE}; \
 	arch=`$(RPM) -qp $${name}-kmod-$${version}.src.rpm --qf %{arch} | tail -1`; \
 	pkg1=kmod-$${name}*$${version}.$${arch}.rpm; \
 	fakeroot $(ALIEN) --bump=0 --scripts --to-deb $$pkg1; \
 	$(RM) $$pkg1
-endif
+
+
+deb-dkms: deb-local rpm-dkms
+	name=${PACKAGE}; \
+	version=${VERSION}-${RELEASE}; \
+	arch=`$(RPM) -qp $${name}-dkms-$${version}.src.rpm --qf %{arch} | tail -1`; \
+	pkg1=$${name}-dkms-$${version}.$${arch}.rpm; \
+	fakeroot $(ALIEN) --bump=0 --scripts --to-deb $$pkg1; \
+	$(RM) $$pkg1
 
 deb-utils: deb-local rpm-utils
-if CONFIG_USER
 	name=${PACKAGE}; \
 	version=${VERSION}-${RELEASE}; \
 	arch=`$(RPM) -qp $${name}-$${version}.src.rpm --qf %{arch} | tail -1`; \
@@ -58,6 +64,5 @@ if CONFIG_USER
 	rmdir $${path_prepend}; \
 	$(RM) $$pkg1 $$pkg2 $$pkg3 $$pkg4 $$pkg5 $$pkg6 $$pkg7 \
 	    $$pkg8 $$pkg9;
-endif
 
-deb: deb-kmod deb-utils
+deb: deb-kmod deb-dkms deb-utils