Revert "Put PID file under DROONGA_BASE_DIR."

This reverts commit a31e1dc7be8dc784348c791c95984a1ac7dd6b5d.
PID file still should be placed under /var/run (or /run).
Create PID file directory by the service startup script.

The directory is always removed by system on shutdown or startup.
We have to prepare it on every startup.