Required callbacks for user & hostname

[martinheidegger]

This PR proposes a fix to an issue/bug in the osenv API.

Both the user and hostname methods can optionally fallback to the executables: whoami and hostname. If a fallback happens, that fallback will be async. In other words, depending on the environment user = osenv.user() may or may-not work as intended. osenv.user((err, user) => {} will always work as intended with the fallback. The important thing here is that the user will not notice the error.

To mitigate this, this PR uses a soft-breaking API. osenv.user() and osenv.hostname() will always return undefined. With a callback it will work, consistently.

An alternative solution to this would be to throw an error if no callback is given and/or write a deprecation warning.

[martinheidegger]

@iarna I updated the description to this PR. I hope it is more explanatory now.