Make PIDLock work on a per-file basis.
Make PIDLock work on a per-file basis.

Now eubin creates a lock file as:

    <conf_dir>/<conf_file>.lockfile

Before this, a process locks entire directory when it start processing.
This turns out too coerce, as it makes almost impossible to access
multiple POP3 maildrops in parallel.

fujimotos/Eubin