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.
Merge branch 'old-system-needs-iconv' (close #169)

Conflicts:
	autoload/vital/__latest__/System/File.vim

And previous commit(4243d6e) message is slightly wrong.
system() returns raw string encoded by system encoding yet (Thanks @k-takata).

-Add decoding: Old system() returns raw string encoded by system encoding.
+Add decoding: system() returns raw string encoded by system encoding.