We avoid abbreviations where possible; rename git_cred to
git_credential.

In addition, we have standardized on a trailing `_t` for enum types,
instead of using "type" in the name.  So `git_credtype_t` has become
`git_credential_t` and its members have become `GIT_CREDENTIAL` instead
of `GIT_CREDTYPE`.

Finally, the source and header files have been renamed to `credential`
instead of `cred`.

Keep previous name and values as deprecated, and include the new header
files from the previous ones.

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.

Custom transports may want to ask libgit2 to invoke a configured
credential or certificate callback; however they likely do not know if a
callback was actually configured.  Return a sentinal value
(GIT_PASSTHROUGH) if there is no callback configured instead of crashing.

As with the callbacks, third-party implementations of smart subtransports cannot
reach into the opaque struct and thus cannot know what options the user set.

Add a getter for these options to copy the proxy options into something external
implementors can use.

We should explicitly include the declaration of git_strarray
from "include/git2/sys/transport.h"