In libgit2 nomenclature, when we need to verb a direct object, we name
a function `git_directobject_verb`.  Thus, if we need to init an options
structure named `git_foo_options`, then the name of the function that
does that should be `git_foo_options_init`.

The previous names of `git_foo_init_options` is close - it _sounds_ as
if it's initializing the options of a `foo`, but in fact
`git_foo_options` is its own noun that should be respected.

Deprecate the old names; they'll now call directly to the new ones.

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.