libgit2 has two distinct requirements that were previously solved by
`git_buf`.  We require:

1. A general purpose string class that provides a number of utility APIs
   for manipulating data (eg, concatenating, truncating, etc).
2. A structure that we can use to return strings to callers that they
   can take ownership of.

By using a single class (`git_buf`) for both of these purposes, we have
confused the API to the point that refactorings are difficult and
reasoning about correctness is also difficult.

Move the utility class `git_buf` to be called `git_str`: this represents
its general purpose, as an internal string buffer class.  The name also
is an homage to Junio Hamano ("gitstr").

The public API remains `git_buf`, and has a much smaller footprint.  It
is generally only used as an "out" param with strict requirements that
follow the documentation.  (Exceptions exist for some legacy APIs to
avoid breaking callers unnecessarily.)

Utility functions exist to convert a user-specified `git_buf` to a
`git_str` so that we can call internal functions, then converting it
back again.

Update the proxy detection for a remote.

1. Honor `http.<url>.proxy` syntax for a remote's direct URL and
   parent URLs.
2. Honor an empty configuration URL to override a proxy configuration.

Add tests to ensure that configuration specificity is honored.

Item 2 of 3 from #4164

Signed-off-by: Mathieu Parent <math.parent@gmail.com>

Attempt authentication when a username is presented but a password is
not; this can happen in particular when users are doing token
authentication and specifying the token in the URL itself.  For example,
`https://token@host/` is a valid URI and should be treated as a username
of `token` with an empty password.

Provide a mechanism for system components to register for initialization
and shutdown of the libgit2 runtime.

When the failure is clearly an auth failure
(as opposed to possibly an auth failure),
use the error code GIT_EAUTH instead of GIT_ERROR.

While we're here, fix a typo and improve an error message.

Fixes #5389.

In commit b9c5b15a7 (http: use the new httpclient, 2019-12-22), the HTTP
code got refactored to extract a generic HTTP client that operates
independently of the Git protocol. Part of refactoring was the creation
of a new `git_http_request` struct that encapsulates the generation of
requests. Our Git-specific HTTP transport was converted to use that in
`generate_request`, but during the process we forgot to set up custom
headers for the `git_http_request` and as a result we do not send out
these headers anymore.

Fix the issue by correctly setting up the request's custom headers and
add a test to verify we correctly send them.

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.