Rename server.py into dpserver and use rc.py

Objective
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- unify excutable name into `dp*`
- use new command-line interface introduced in `dpmanip`

Summary
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- introduce another configure loader in which configure is loaded into argparser instance
- rename `server.py` into `dpserver`
- change CLI of `dpserver`

PR status
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- related to #125 
This is positional argument.
In the case satisfying the following three conditions:

- `allow_empty` is True
- cannot read value from configure file (`try:` fails)
- default value is not set

a positional argument is appended into `parser`.