Since each cred defines the username on their own, introduce
git_cred__username to retrieve the username pointer from them.

This exists as ssh needs to know about the username to use before it can
query for the supported authentication methods.

Introduce this error code to signal an authentication failure.

Add docs for git_clone_local_t and move the docs for the
git_clone_options to each field.

Bump library version to 0.21.0 and SONAME to 21

Since the SOVERSION doesn't need to follow the library's version and
simply needs to be monotonically increasing whenever we release
something that breaks the ABI, we can set some number and allow multiple
versions of the library to be installed side-by-side.

We start here with the minor version as that's what we release for now,
and it allows to backport this change to earlier versions.

This was a bad idea. Don't free except in the free function.

There is no reason why we need to use a callback here. A string array
fits better with the usage, as this is not an event and we don't need
anything from the user.