Fix systemd_ RPM macros usage on Debian-based distributions

Debian-based distributions do not seem to provide RPM macros for
dealing with systemd pre- and post- (un)install actions: this results
in errors when installing or upgrading .deb packages because the
resulting control scripts contain the following unresolved macros:

 * %systemd_post
 * %systemd_preun
 * %systemd_postun

Fix this by providing default values for postinstall, preuninstall and
postuninstall scripts when these macros are not defined.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes openzfs#7074 
Closes openzfs#7100

rpm/generic/zfs.spec.in

@@ -245,8 +245,15 @@ find %{?buildroot}%{_libdir} -name '*.la' -exec rm -f {} \;
 
 %post
 %if 0%{?_systemd}
+%if 0%{?systemd_post:1}
 %systemd_post %{systemd_svcs}
 %else
+if [ "$1" = "1" -o "$1" = "install" ] ; then
+    # Initial installation
+    systemctl preset %{systemd_svcs} >/dev/null || true
+fi
+%endif
+%else
 if [ -x /sbin/chkconfig ]; then
     /sbin/chkconfig --add zfs-import
     /sbin/chkconfig --add zfs-mount
@@ -258,9 +265,17 @@ exit 0
 
 %preun
 %if 0%{?_systemd}
+%if 0%{?systemd_preun:1}
 %systemd_preun %{systemd_svcs}
 %else
-if [ "$1" = "0" ] && [ -x /sbin/chkconfig ]; then
+if [ "$1" = "0" -o "$1" = "remove" ] ; then
+    # Package removal, not upgrade
+    systemctl --no-reload disable %{systemd_svcs} >/dev/null || true
+    systemctl stop %{systemd_svcs} >/dev/null || true
+fi
+%endif
+%else
+if [ "$1" = "0" -o "$1" = "remove" ] && [ -x /sbin/chkconfig ]; then
     /sbin/chkconfig --del zfs-import
     /sbin/chkconfig --del zfs-mount
     /sbin/chkconfig --del zfs-share
@@ -271,7 +286,11 @@ exit 0
 
 %postun
 %if 0%{?_systemd}
+%if 0%{?systemd_postun:1}
 %systemd_postun %{systemd_svcs}
+%else
+systemctl --system daemon-reload >/dev/null || true
+%endif
 %endif
 
 %files