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.c

@@ -2,6 +2,9 @@
 
 #include "config.h"
 
+#include <sys/types.h>
+#include <sys/wait.h>
+
 #include "shell-util.h"
 #include <glib/gi18n-lib.h>
 #include <gtk/gtk.h>
@@ -861,3 +864,26 @@ shell_session_is_active_for_systemd (void)
   return TRUE;
 #endif
 }
+
+/**
+ * shell_util_wifexited:
+ * @status: the status returned by wait() or waitpid()
+ * @exit: (out): the actual exit status of the process
+ *
+ * Implements libc standard WIFEXITED, that cannot be used JS
+ * code.
+ * Returns: TRUE if the process exited normally, FALSE otherwise
+ */
+gboolean
+shell_util_wifexited (int  status,
+                      int *exit)
+{
+  gboolean ret;
+
+  ret = WIFEXITED(status);
+
+  if (ret)
+    *exit = WEXITSTATUS(status);
+
+  return ret;
+}