Process, System.File: old system() needs iconv()

* Add encoding: Old system() assumes argument's command is encoded by system
  encoding (does not encode its own).
* Add decoding: Old system() returns raw string encoded by system encoding.

This behavior was changed in 7.4.122:
  https://groups.google.com/forum/#!searchin/vim_dev/7.4.122/vim_dev/KXzoTZz3-MY/HiyvvjPWsssJ

cf.
* Issue #126
* Commit 8a98fc562370c5e9de2b6b44cb786ad12c206336
* Patch 7.4.122
  https://groups.google.com/forum/#!searchin/vim_dev/7.4.122/vim_dev/KXzoTZz3-MY/HiyvvjPWsssJ

Thanks @k-takata to point out this problem.