GnuTLS: Remove support for versions older than 2.12.x

GnuTLS: Remove support for versions older than 2.12.x

GnuTLS project has marked 2.12.x obsolete since January 2014. There is
not much need for maintaining support for obsolete versions of the
library, so drop all #if/#endif blocks targeting 2.x.y versions. In
practice, none of these were requiring 2.12.x version with x greater
than 0, so 2.12.x remains supported for now.

In addition, add newer version (GnuTLS 3.0.18 and newer) to fetch client
and server random from the session since the old method is not supported
by new GnuTLS versions and as such, gets removed with rest of the old
ifdef blocks.

Signed-off-by: Jouni Malinen <j@w1.fi>