Make some syntax change to follow coding style.

fixes #6003

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.