auth: rework polkit autorization to use DBUS interface directly

This makes NetworkManager independent of <polkit/polkit.h>
development headers and libpolkit-gobject-1.so library.
Instead communicate directly with polkit using its DBUS
interface.

PolicyKit support is now always compiled in. You can control
polkit authorization with the configuration option
  [main]
  auth-polkit=yes|no

If the configure option is omitted, a build time default
value is used. This default value can be set with the
configure option --enable-polkit.

This commit adds a new class NMAuthManager that reimplements the
relevant DBUS client parts. It takes source code from the polkit
library.

https://bugzilla.gnome.org/show_bug.cgi?id=734146

Signed-off-by: Thomas Haller <thaller@redhat.com>

src/nm-active-connection.c

@@ -319,7 +319,7 @@ nm_active_connection_get_user_requested (NMActiveConnection *self)
 {
 	g_return_val_if_fail (NM_IS_ACTIVE_CONNECTION (self), FALSE);
 
-	return !nm_auth_subject_get_internal (NM_ACTIVE_CONNECTION_GET_PRIVATE (self)->subject);
+	return nm_auth_subject_is_unix_process (NM_ACTIVE_CONNECTION_GET_PRIVATE (self)->subject);
 }
 
 NMDevice *