Add API functions to libpq to interrogate SSL related stuff.

This makes it possible to query for things like the SSL version and cipher
used, without depending on OpenSSL functions or macros. That is a good
thing if we ever get another SSL implementation.

PQgetssl() still works, but it should be considered as deprecated as it
only works with OpenSSL. In particular, PQgetSslInUse() should be used to
check if a connection uses SSL, because as soon as we have another
implementation, PQgetssl() will return NULL even if SSL is in use.