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.
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.
