Force preset of systemd-resolved on package upgrade

Just changing /etc/nsswitch.conf is pointless without this.

systemd.spec

@@ -621,6 +621,18 @@ if [ $1 -eq 0 ] ; then
                 >/dev/null || :
 fi
 
+%triggerun -- systemd < 246~rc2-2
+# This is for upgrades from previous versions before systemd-resolved became the default.
+systemctl --no-reload preset systemd-resolved.service &>/dev/null || :
+
+grep -q 'Generated by NetworkManager' /etc/resolv.conf 2>/dev/null && \
+  echo -e '/etc/resolv.conf was generated by NetworkManager.\nConsider removing it to let systemd-resolved manage this file.' \
+  || :
+
+if systemctl is-enabled systemd-resolved.service &>/dev/null; then
+  systemctl start systemd-resolved.service &>/dev/null;
+fi
+
 %post libs
 %{?ldconfig}
 
@@ -776,6 +788,9 @@ fi
 %changelog
 * Sun Jul 26 2020 Zbigniew Jędrzejewski-Szmek <zbyszek@in.waw.pl> - 246~rc2-2
 - Make /tmp be 50% of RAM again (#1856514)
+- Re-run 'systemctl preset systemd-resolved' on upgrades.
+  /etc/resolv.conf is not modified, by a hint is emitted if it is
+  managed by NetworkManager.
 
 * Fri Jul 24 2020 Zbigniew Jędrzejewski-Szmek <zbyszek@in.waw.pl> - 246~rc2-1
 - New pre-release with incremental fixes