getinfo: access SSL internals via Curl_ssl

In the ongoing endeavor to abstract out all SSL backend-specific
functionality, this is the next step: Instead of hard-coding how the
different SSL backends access their internal data in getinfo.c, let's
implement backend-specific functions to do that task.

This will also allow for switching SSL backends as a runtime option.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>