Sync std{in,out,err} Encodings with `Terminal`

This is a spin off from [#11168](https://github.com/ariya/phantomjs/pull/11168#issuecomment-15792650).

This modification keeps the `system.std{in,out,err}` encodings in sync with the `Terminal`.  That is, whenever the `Terminal` encoding gets changed, that update will now be propagated to the `System` module.

*SIDE NOTE:*  Should the reverse case (propagate `system.std{in,out,err}` encoding change to `Terminal`) be handled?