Packages: fixed building for Ubuntu 16.04 "xenial".

Changes introduced in a27532e3a17b effectively broke building of the unit
package due to missed dh_installsystemd script in older debhelper 9.x.

Once Ubuntu 16.04 reach EOL, the following actions should be made:
 - this commit should be reverted;
 - minimal debhelper version should be increased to 11.

pkg/deb/debian/rules.in

@@ -10,6 +10,7 @@ DPKG_EXPORT_BUILDFLAGS = 1
 include /usr/share/dpkg/buildflags.mk
 
 DEB_HOST_MULTIARCH ?= $(shell dpkg-architecture -qDEB_HOST_MULTIARCH)
+CODENAME := $(shell lsb_release -cs)
 
 BUILDDIR_unit = $(CURDIR)/debian/build-unit
 BUILDDIR_unit_debug = $(CURDIR)/debian/build-unit-debug
@@ -96,7 +97,11 @@ install: build do.tests
 	dh_testroot
 	dh_prep
 	dh_installdirs
+ifeq ($(CODENAME), xenial)
+	dh_installinit
+else
 	dh_installsystemd
+endif
 	dh_installlogrotate
 	cd $(BUILDDIR_unit) && DESTDIR=$(INSTALLDIR) make install
 	cd $(BUILDDIR_unit) && DESTDIR=$(INSTALLDIR_dev) make libunit-install

pkg/deb/debian/unit.init

@@ -0,0 +1,74 @@
+#!/bin/sh
+#
+# unitd              NGINX Unit
+#
+### BEGIN INIT INFO
+# Provides:          unitd
+# Required-Start:    $network $remote_fs
+# Required-Stop:     $network $remote_fs
+# Default-Start:     2 3 4 5
+# Default-Stop:      0 1 6
+# Short-Description: NGINX Unit
+# Description:       NGINX Unit
+### END INIT INFO
+PATH=/sbin:/bin:/usr/sbin:/usr/bin
+DAEMON=/usr/sbin/unitd
+NAME=unit
+DESC=unitd
+
+#includes lsb functions
+. /lib/lsb/init-functions
+
+test -f $DAEMON || exit 0
+
+umask 022
+
+# Read configuration variable file if it is present
+[ -r /etc/default/$NAME ] && . /etc/default/$NAME
+
+case "$1" in
+    start)
+       	log_daemon_msg "Starting $DESC" "$NAME"
+        if start-stop-daemon --start --quiet --pidfile /var/run/$NAME.pid \
+                --exec $DAEMON -- $DAEMON_ARGS; then
+                log_end_msg 0
+        else
+                log_end_msg 1
+        fi
+        ;;
+    status)
+        status_of_proc -p "$PIDFILE" "$DAEMON" "$NAME" && exit 0 || exit $?
+        ;;
+    stop)
+        log_daemon_msg "Stopping $DESC" "$NAME"
+        if start-stop-daemon --oknodo --stop --quiet --pidfile /var/run/$NAME.pid \
+                --exec $DAEMON; then
+                log_end_msg 0
+        else
+                log_end_msg 1
+        fi
+        ;;
+   reload|force-reload)
+        echo "Not implemented." >&2
+        exit 1
+        ;;
+    restart)
+        log_action_begin_msg "Restarting $DESC" "$NAME"
+
+        start-stop-daemon --stop --quiet --pidfile \
+                /var/run/$NAME.pid --exec $DAEMON || true
+        sleep 1
+        if start-stop-daemon --start --quiet --pidfile \
+                /var/run/$NAME.pid --exec $DAEMON -- $DAEMON_ARGS; then
+                log_end_msg 0
+        else
+                log_end_msg 1
+        fi
+        ;;
+    *)
+        echo "Usage: /etc/init.d/$NAME {start|status|stop|restart|reload|force-reload}" >&2
+        exit 1
+        ;;
+esac
+
+exit 0