Serverinfo

This adds support for so-called "serverinfo" extensions to OpenSSL.

According to this patch, a "serverinfo" extension is an empty ClientHello extension that the server can respond to.  The client can specify arbitrary extension numbers, and the server can specify arbitrary extension responses via a PEM file.

This can be used to implement Certificate Transparency's SignedCertificateTimestampList, as well as TACK's TackExtension.  It could be used by any other proposal that want to staple server info into the handshake.

This patch should be complete, and includes some light tests.  If this is accepted, I could follow-on with improved tests and docs.