If the libcurl stream is available, use that as the underlying stream
instead of the socket stream. This allows us to set a proxy for HTTPS
connections.
The read and write callbacks passed to SSLSetIOFuncs() have been
rewritten to match the implementation used on opensource.apple.com and
other open source projects like VLC.
This change also fixes a bug where the read callback could get into
an infinite loop when 0 bytes were read.
Do not automatically fail on a bad certificate, but let the caller
decide. This means we don't need our switch on errors anymore but can
return a string representation from the security framework.
