irnetproxy.py: Cleanup: Remove unnecessary argparse `dest` arguments
irnetproxy.py: Cleanup: Remove unnecessary argparse `dest` arguments

http://docs.python.org/2.7/library/argparse.html#dest says:

> ArgumentParser generates the value of dest by taking the first long
> option string and stripping away the initial `--` string. Any internal
> `-` characters will be converted to `_` characters.

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