honour dead process with existing pidfile

We need to consider situations where the process is no longer running but the pidfile exists and eventually clean it up.

```
[varnish-mmahut.rhcloud.com 53f47c48e0b8cde28d000b7c]\> gear start
Starting gear...
Message from dlopen:
Not running as root, no priv-sep
[varnish-mmahut.rhcloud.com 53f47c48e0b8cde28d000b7c]\> cat ${OPENSHIFT_VARNISH_DIR}var/run/varnish.pid
311773
[varnish-mmahut.rhcloud.com 53f47c48e0b8cde28d000b7c]\> kill -9 311773
[varnish-mmahut.rhcloud.com 53f47c48e0b8cde28d000b7c]\> gear start
Starting gear...
CLIENT_ERROR: Varnish is dead but pid file exists
[varnish-mmahut.rhcloud.com 53f47c48e0b8cde28d000b7c]\> gear stop
Stopping gear...
[varnish-mmahut.rhcloud.com 53f47c48e0b8cde28d000b7c]\> gear start
Starting gear...
Message from dlopen:
Not running as root, no priv-sep
[varnish-mmahut.rhcloud.com 53f47c48e0b8cde28d000b7c]\>
```
This is not sufficient. Checking for the existing of a directory with a matching PID does not guarantee that that pid is 1) owned by the user, or 2) a varnish process.