Inject user-emacs-directory' in async-start'

[brabalan]

This should address issue #3390 .

[syl20bnr]

@brabalan @justbur is this PR still required or did you manage to resolve the issue differently ?

[justbur]

I think @brabalan's reason for needing this is somewhat unique (running two separate instances of emacs side-by-side, one of which is spacemacs), and this is not necessarily a robust solution. However, I don't think it hurts anyone to do this.

We talked about other possible solutions, but haven't found any yet that satisfies @brabalan's use case.

[brabalan]

@syl20bnr I still need this to be able to run spacemacs alongside another instance of emacs. This works for me, but I would be happy with any other solutions that lets me do that.

[AdrieanKhisbe]

I'm facing the same use case than @brabalan,
The patch is very clean and explicit and would not cause trouble to people having just one .emacs.d

Shall we merge it? :)

[TheBB]

Indeed, let's.

Thanks! Cherry-picked in develop. You can safely delete your branch.