curl: modernize usage of Secure Transport and OpenSSL

The openssl that ships with OS X does not support TLS 1.1 or 1.2. This
is a security issue for for applications that use functionality from
libssl.

On 10.8 and newer, Apple has deprecated use of openssl and added support
for TLS 1.1 and 1.2 to its Secure Transport framework (or "darwinssl" in
curl). On older versions of OS X, a newer openssl is required to obtain
such functionality.

Thus, we default to using darwinssl where it makes sense. An option to
use Homebrew's openssl is provided. On platforms where Secure Transport
does not support the newer protocols, we simply use Homebrew's openssl.

Closes #25824.