NetworkAgent: add support for VPN connections

VPN secrets are stored by the plugins, that provide separate
helpers for authentication. This commit adds the support for invoking
the binaries and pass them connection details.
For plugins that support it (as exposed by their keyfile), we invoke
them in "external-ui-mode" and expect a set of metadata about the
secrets which is used to build a shell styled dialog.

https://bugzilla.gnome.org/show_bug.cgi?id=658484

src/shell-util.h

@@ -52,6 +52,9 @@ void shell_shader_effect_set_double_uniform (ClutterShaderEffect *effect,
 
 gboolean shell_session_is_active_for_systemd (void);
 
+gboolean shell_util_wifexited                  (int               status,
+                                                int              *exit);
+
 G_END_DECLS
 
 #endif /* __SHELL_UTIL_H__ */