save old config.ini during .deb instalation

build-deb/luser/DEBIAN/postinst

@@ -1,3 +1,5 @@
 #!/bin/sh
 /usr/bin/systemctl enable luser.service
 /sbin/service luser start
+cp /tmp/oldluserconfig.ini /var/luser/luser/config.ini
+rm /tmp/oldluserconfig.ini

build-deb/luser/DEBIAN/preinst

@@ -0,0 +1,2 @@
+#!/bin/sh
+cp /var/luser/luser/config.ini /tmp/oldluserconfig.ini

build-deb/luser/DEBIAN/prerm

@@ -1,3 +1,4 @@
 #!/bin/sh
 /sbin/service luser stop
 /usr/bin/systemdctl disable luser.service
+cp /var/luser/luser/config.ini /tmp/oldluserconfig.ini