networkmanager: Add rc-manager option

Add an option to set the rc-manager parameter in NetworkManager.conf,
which controls how NetworkManager handles resolv.conf. This sets the
default rc-manager to "resolvconf", which solves #61490. It
additionally allows the user to change rc-manager without interference
from configuration activations.

nixos/modules/services/networking/networkmanager.nix

@@ -17,6 +17,7 @@ let
     plugins=keyfile
     dhcp=${cfg.dhcp}
     dns=${cfg.dns}
+    rc-manager=${cfg.rc-manager}
 
     [keyfile]
     ${optionalString (cfg.unmanaged != [])
@@ -283,6 +284,25 @@ in {
         '';
       };
 
+      rc-manager = mkOption {
+        type = types.enum [ "symlink" "file" "resolvconf" "netconfig" "unmanaged" "none" ];
+        default = "resolvconf";
+        description = ''
+          Set the <literal>resolv.conf</literal> management mode.
+          </para>
+          <para>
+          A description of these modes can be found in the main section of
+          <link xlink:href="https://developer.gnome.org/NetworkManager/stable/NetworkManager.conf.html">
+            https://developer.gnome.org/NetworkManager/stable/NetworkManager.conf.html
+          </link>
+          or in
+          <citerefentry>
+            <refentrytitle>NetworkManager.conf</refentrytitle>
+            <manvolnum>5</manvolnum>
+          </citerefentry>.
+        '';
+      };
+
       dispatcherScripts = mkOption {
         type = types.listOf (types.submodule {
           options = {