gpg-agent plugin service check changed

Running on OsX 10.8:

When using gpg-agent plugin, found numerous instances of gpg-agent running on my system, growing linearly with time.  Found existing check for running services is calling a deprecated command.

gpg-agent -q has the same effect.

![image](https://f.cloud.github.com/assets/191587/1030808/d4cfe8e6-0ec5-11e3-8290-c8ccb23313bb.png)