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.