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Give callers the ability to select how to handle redirects - either
supporting redirects during the initial connection (so that, for
example, `git.example.com/repo` can redirect to `github.com/example/repo`)
or all/no redirects.  This is for compatibility with git.

The existing mechanism for providing options to remote fetch/push calls,
and subsequently to transports, is unsatisfactory.  It requires an
options structure to avoid breaking the API and callback signatures.

1. Introduce `git_remote_connect_options` to satisfy those needs.

2. Add a new remote connection API, `git_remote_connect_ext` that will
   take this new options structure.  Existing `git_remote_connect` calls
   will proxy to that.  `git_remote_fetch` and `git_remote_push` will
   proxy their fetch/push options to that as well.

3. Define the interaction between `git_remote_connect` and fetch/push.
   Connect _may_ be called before fetch/push, but _need not_ be.  The
   semantics of which options would be used for these operations was
   not specified if you specify options for both connect _and_ fetch.
   Now these are defined that the fetch or push options will be used
   _if_ they were specified.  Otherwise, the connect options will be
   used if they were specified.  Otherwise, the library's defaults will
   be used.

4. Update the transports to understand `git_remote_connect_options`.
   This is a breaking change to the systems API.

Make some syntax change to follow coding style.

This change makes `git_remote_name_is_valid` be part of the public
interface of the library. This is needed for other language bindings to
be able to find this symbol (like in git2go, when linking against
libgit2 dynamically).

Using a callback to set a resolve_url is not particularly idiomatic.
Deprecate it in favor of the `set_instance_url` and
`set_instance_pushurl` functions which can now be called from the
`git_remote_ready_cb` callback.

Introduce a new callback that fires when the remote is ready to connect.

Users may want to override the URL on a particular instance of a remote,
instead of updating the configuration.  Previously, users could use a
callback to do this, but this is not particularly idiomatic.