The old interface only allowed one to perform terminal detection on a
handle and change it to UTF-8 if it was not a terminal. Turns our, it
makes sense to sometimes leave the original encoding on a handle even if
it does not point to a terminal.
* Change the interface to separate these two use-cases.
* Extract main-wrappers to a new `Main` module.
This function allows one to not only safely set the best encoding on a
file descriptor, but also restore the previous one. The idea is that it
will be used on file descriptors that come from the outside and so we
don’t know how they were used and will be used after us.
