Since libssh2 doesn't read host configuration from the config file,
this callback can be used to hand over URL resolving to the client
without touching the SSH implementation itself.

For consistency with other "type" enums, rename
git_remote_completion_type to git_remote_completion_t.

The `git_push_transfer_progress` is a callback and as such should be
suffixed with `_cb` for consistency.  Rename
`git_push_transfer_progress` to `git_push_transfer_progress_cb`.

Update internal usage of `git_transfer_progress` to
`git_indexer_progreses`.

Since we now always build the library with cdecl calling conventions,
our callbacks should be decorated as such so that users will not be able
to provide callbacks defined with other calling conventions.

The `GIT_CALLBACK` macro will inject the `__cdecl` attribute as
appropriate.

This supersedes the functionality of remote_create_with_fetchspec, remote_create_anonymous and remote_create_detached.