Introduce wp_http_supports as a much less hacky replacement for the...

Introduce wp_http_supports as a much less hacky replacement for the http_transport_(get|post)_debug hooks that plugins could have 
been using to detect if things like ssl requests were working.
See #17251 props mdawaffe


git-svn-id: https://core.svn.wordpress.org/trunk@17914 1a063a9b-81f0-0310-95a4-ce76da25c4cd