dbus: Remove 'at_console', add 'netdev' settings

The 'at_console' D-Bus policy setting has been deprecated for more then
10 years and could be ignored at any time in the future. Moreover, while
the intend was to allow locally logged on users to interact with iwd, it
didn't actually do that.
More info at https://www.spinics.net/lists/linux-bluetooth/msg75267.html
and https://gitlab.freedesktop.org/dbus/dbus/-/issues/52
Therefor remove the 'at_console' setting block.

On Debian (based) systems, there is a standard defined group which is
allowed to manage network interfaces, and that is the 'netdev' group.
So add a D-Bus setting block to grant the 'netdev' group that access.

src/iwd-dbus.conf

@@ -18,7 +18,7 @@
     <allow send_interface="net.connman.iwd.Agent"/>
   </policy>
 
-  <policy at_console="true">
+  <policy group="netdev">
     <allow send_destination="net.connman.iwd"/>
     <allow send_interface="net.connman.iwd.Agent"/>
   </policy>