When compiling libgit2 with -DDEPRECATE_HARD, we add a preprocessor
definition `GIT_DEPRECATE_HARD` which causes the "git2/deprecated.h"
header to be empty. As a result, no function declarations are made
available to callers, but the implementations are still available to
link against. This has the problem that function declarations also
aren't visible to the implementations, meaning that the symbol's
visibility will not be set up correctly. As a result, the resulting
library may not expose those deprecated symbols at all on some platforms
and thus cause linking errors.

Fix the issue by conditionally compiling deprecated functions, only.
While it becomes impossible to link against such a library in case one
uses deprecated functions, distributors of libgit2 aren't expected to
pass -DDEPRECATE_HARD anyway. Instead, users of libgit2 should manually
define GIT_DEPRECATE_HARD to hide deprecated functions. Using "real"
hard deprecation still makes sense in the context of CI to test we don't
use deprecated symbols ourselves and in case a dependant uses libgit2 in
a vendored way and knows it won't ever use any of the deprecated symbols
anyway.

We've accumulated quite some functions which are never used outside of
their respective code unit, but which are lacking the `static` keyword.
Add it to reduce their linkage scope and allow the compiler to optimize
better.

We _dispose_ the contents of objects; we _free_ objects (and their
contents).  Update `git_strarray_free` to be `git_strarray_dispose`.
`git_strarray_free` remains as a deprecated proxy function.

cred: change enum to git_credential_t and GIT_CREDENTIAL_*

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.

Stop returning a void for functions, future-proofing them to allow them
to fail.

In "remote.c", we have a chunk of code that is #ifdef'fed out via
`#if 0` with a comment that we could export it as a helper function.
The code was implemented in 2013 and ifdef'fed in 2014, which shows that
there's clearly no interest in having such a helper at all.

As this block has recently created some confusion about `p_getenv` due
to it containing the only reference to that function in our codebase,
let's remove this block altogether.

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.

Since libssh2 doesn't read host configuration from the config file,
this callback can be used to hand over URL resolving to the client
without touching the SSH implementation itself.

Update internal usage of `git_transfer_progress` to
`git_indexer_progreses`.