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.

As refdb and odb backends can be allocated by client code, libgit2
can’t know whether an alternative memory allocator was used, and thus
should not try to call `git__free` on those objects.

Instead, odb and refdb backend implementations must always provide
their own `free` functions to ensure memory gets freed correctly.

Resolve documentation warnings

Removing a reflog upon ref deletion is something which only some
backends might wish to do. Backends which are database-backed may wish
to archive a reflog, log-based ones may not need to do anything.

A transaction allows you to lock multiple references and set up changes
for them before applying the changes all at once (or as close as the
backend supports).

This can be used for replication purposes, or for making sure some
operations run when the reference is locked and thus cannot be changed.

There were a couple of "init_opts()" functions a few more cases
of structure initialization that I somehow missed.

The basic structure of each function is courtesy of arrbee.

We don't actually pass the old value yet.

Add a parameter to the backend to allow checking for the old symbolic
target.