Packages: removed support for EOL'ed Debian/Ubuntu distros.

While here, default debhelper compat level bumped to 11 (this is the
version installed out of the box on Ubuntu 18.04 "bionic" which is
the oldest one from supported distros).

pkg/deb/debian/rules.in

@@ -99,11 +99,7 @@ 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