Revert "Put PID file under DROONGA_BASE_DIR."

This reverts commit 5c7025783d824ad106ec5cd5a4ee956d04bf4627.
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.