[RFC] Learn "--server location" command line option.

Partially related to pull #1277--it would be convenient to have an alternative to `$NVIM_LISTEN_ADDRESS`, and possibly replace it. However, I don't yet know the precise strategy that should be used, so right now I just want to propose a way to override it.

As an example, I use nvim as a git editor, but when I `:!git commit`, I see "Failed to start server: address in use", so to work without error, I have to configure git like this:
```bash
$ git config --global core.editor 'env NVIM_LISTEN_ADDRESS=/tmp/git-nvim nvim'
```
I'd rather:
```bash
$ git config --global core.editor 'nvim --server /tmp/git-nvim'
```
Can we set the env var later if error == 0? Otherwise this will be set even when the socket is not there.