Defer dlopen until it's needed when dynamically loading OpenSSL
libraries.

Provide an interface around OpenSSL to dynamically load the libraries
and symbols, so that users can distribute a libgit2 library that is not
linked directly against OpenSSL.  This enables users to target multiple
distributions with a single binary.

This mechanism is optional and disabled by default.  Configure cmake
with -DUSE_HTTPS=OpenSSL-Dynamic to use it.

Refactor the OpenSSL stream implementation so that the legacy code is better
abstracted.  This will enable future development.

During valgrind runs, we try to swap out the OpenSSL allocators for our
own.  This allows us to avoid some unnecessary warnings about usage.
Unfortunately, many builds of OpenSSL do not allow you to swap
allocators; for example FIPS builds and the builds running in CentOS.

Try to swap the allocators, but do not fail when they cannot be
customized.

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

Move the settings global data teardown into its own separate function,
instead of intermingled with the global state.

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're missing some header includes leading to missing function
prototypes. While we currently don't warn about these, we should have
their respective headers included in order to detect the case where a
function signature change results in an incompatibility.