Stash apply: stage new files even when not updating the index

Rename FALLBACK to UNSPECIFIED

Don't allow growing borrowed buffers

Fallback describes the mechanism, while unspecified explains what the
user is thinking.

We've been using EINVALIDSPEC for a while to mean this, but that name
is too specific. Introduce this to be more explicit.

We currently recommend using `git_buf_grow` in order to make a buffer
make an owned copy of the memory it points to. This is not behaviour we
should encourage, so remove this recommendation.

The function itself is not changed, as we need to remain compatible, but
it will be changed not to allow usage on borrowed buffers.

Remove run-time configuration settings from submodules

The information is exposed by curl for some crypto libraries in the form
of name:content strings. We can't do much more than return this
information.

If the stream claims to support this feature, we can let the transport
set the proxy.

We also set HTTPPROXYTUNNEL option so curl can create a tunnel through
the proxy which lets us create our own TLS session (if needed).