Partial fix for ticket #2489 -- problems with SNI SSL virtual host certificate valida...

Partial fix for ticket #2489 -- problems with SNI SSL virtual host certificate validation.

Two prongs here:
* We attempt to enable SNI on the SSL stream context with the appropriate hostname... This requires PHP 5.3.2 and OpenSSL that supports the TLS extensions. Unfortunately this doesn't seem to be working in my testing.
* If set $config['http']['curl'] = true, we'll use the CURL backend if available. In my testing on Ubuntu 10.04, this works. No guarantees on other systems.

I'm not enabling CURL mode by default just yet; want to make sure there's no other surprises.