Run docurium as part of the build.  The goal of this is to be able to
evaluate the documentation in a given pull request; as such, this does
not implement any sort of deployment pipeline.

This will allow us to download a snapshot of the documentation from the
CI build and evaluate the docs for a particular pull request; before
it's been merged.

index: preserve extension parsing errors

Deprecate functions and constants more gently

Ensure that we do not use any deprecated functions in the library
source, test code or examples.

Users can define `GIT_DEPRECATE_HARD` if they want to remove all
functions that we've "softly" deprecated.

Move the deprecated stream tests into their own compilation unit.  This
will allow us to disable any preprocessor directives that apply to
deprecation just for these tests (eg, disabling `GIT_DEPRECATED_HARD`).

Avoid the deprecated `git_stream_cb` typedef since we want to compile
the library without deprecated functions or types.  Instead, we can
unroll the alias to its actual type.

Add `@deprecated` to the functions that are, so that they'll appear that
way in docurium.

`git_stream_register_tls` is now deprecated; mark it in an if guard with
the deprecation.

This should not be included in `deprecated.h` since it is an uncommonly
used `sys` header file.

Don't use deprecated constants

Follow up for PR #4917.

Signed-off-by: Sven Strickroth <email@cs-ware.de>

Previously, we would clobber any extension-specific error message with
an "extension is truncated" message. This makes `read_extension`
correctly preserve those errors, takes responsibility for truncation
errors, and adds a new message with the actual extension signature for
unsupported mandatory extensions.

Fix VS warning C4098: 'giterr_set_str' : void function returning a value

Signed-off-by: Sven Strickroth <email@cs-ware.de>

Move `giterr` to `git_error`

Update GITERR and giterr usages in the static code analysis tools to use
the new names.

Move to the `git_error` name in the internal API for error-related
functions.

Replace the `GITERR` values with a `const int` to deprecate error
values.

Move to the `git_error` name in error-related functions, deprecating the
`giterr` functions.  This means, for example, that `giterr_last` is now
`git_error_last`.  The old names are retained for compatibility.

This only updates the public API; internal API and function usage
remains unchanged.

odb: Fix odb foreach to also close on positive error code

In include/git2/odb.h it states that callback can also return
positive value which should break looping.

Implementations of git_odb_foreach() and pack_backend__foreach()
did not respect that.

repository: free memory in symlink detection function

ci: update poxyproxy, run in quiet mode

Update the proxy so that we can enable a quiet mode.

Add/multiply with overflow tweaks

Improve deprecation of old enums

Correct the documentation on the fallback add/multiply with overflow
functions.

Windows provides <intsafe.h> which provides "performant" add and
multiply with overflow operations.  Use them when possible.

Use the smallest unsigned type that is equivalent to `size_t` to
simplify the conditionals.  Error if we're on a system that we believe
offers builtins but we cannot determine which one to use.

The C standard does not specify whether an enum is a signed or unsigned
type.  Obviously, any enum that includes negative values _must_ be
signed, but if all values are positive then the compiler is free to
choose signed or unsigned.

Thus, by changing the type signatures to `git_object_t` and declaring
the old `GIT_OBJ_` values as a signed or unsigned int, we risk a
mismatch between what the compiler has chosen for a `git_object_t`'s
type and our type declaration.

Thus, we declare the deprecated values as the enum instead of guessing.

Recent GCC enables `-Wunused-const-variables`, which makes output quite
noisy.  Disable unused warnings for our deprecated variables.