Remove the configuration state we keep in the remote

We're down to simply having it be a call to create_internal() so let's
simply do that. The rest of the code is just a distraction.

It has now become a no-op, so remove the function and all references to
it.

The base refspecs changing can be a cause of confusion as to what is the
current base refspec set and complicate saving the remote's
configuration.

Change `git_remote_add_{fetch,push}()` to update the configuration
instead of an instance.

This finally makes `git_remote_save()` a no-op, it will be removed in a
later commit.

This is another option which we should not be keeping in the remote, but
is specific to each particular operation.

While this will rarely be different from the default, having it in the
remote adds yet another setting it has to keep around and can affect its
behaviour. Move it to the options.

Instead of having it set in a different place from every other callback,
put it the main structure. This removes some state from the remote and
makes it behave more like clone, where the constructors are passed via
the options.

Add a prune setting in the fetch options to allow to fall back to the
configuration (the default) or to set it on or off.

As a first step in removing the repository-saving logic, don't allow
chaning the url or push url from a remote object, but change the
configuration on the configuration immediately.

Having the setting be different from calling its actions was not a great
idea and made for the sake of the wrong convenience.

Instead of that, accept either fetch options, push options or the
callbacks when dealing with the remote. The fetch options are currently
only the callbacks, but more options will be moved from setters and
getters on the remote to the options.

This does mean passing the same struct along the different functions but
the typical use-case will only call git_remote_fetch() or
git_remote_push() and so won't notice much difference.

The push object knows which remote it's associated with, and therefore
does not need to keep its own copy of the callbacks stored in the
remote.

Remove the copy and simply access the callbacks struct within the
remote.

Centralizing all IO buffer size values

Stash apply

Allow the baseline to be specified as an index, so that users
need not write their index to a tree just to checkout with that
as the baseline.

Adjusting stream buffer size to 64KB

64K is optimal buffer size per https://technet.microsoft.com/en-us/library/cc938632.aspx

Android build doesn't need deps/regex

Ensure frameworks are mentioned in libgit2.pc

When building on Mac OS X, the `CoreFoundation` and `Security` frameworks where missing from `Libs.private` in the generated `libgit2.pc` file.