debian: install policykit-1 or polkitd depending on the version

Old Debian and Ubuntu versions didn't have a separate polkitd yet. Make
both optional so it's installed the one that is available.

contrib/debian/REQUIRED_PACKAGES

@@ -71,7 +71,6 @@ install \
     meson \
     mobile-broadband-provider-info \
     pkg-config \
-    polkitd \
     ppp \
     ppp-dev \
     python3-dbus \
@@ -87,6 +86,8 @@ install \
 
 install_ignore_missing \
     python-setuptools \
+    policykit-1 \
+    polkitd \
     \
     #end