debian: do not stop services until after the package upgrade has been completed

In addition to minimize re6st downtime, this fixes upgrade from
versions that still use init scripts.

When we drop support for Debian 8, we'll increase debian/compat
and we'll revert this commit.

debian/common.mk

@@ -17,3 +17,7 @@ endif
 
 override_dh_install:
 	make DESTDIR=$(TMP) PREFIX=/usr PYTHON=/usr/bin/python install
+
+# BBB: compat < 10
+override_dh_systemd_start:
+	dh_systemd_start --restart-after-upgrade