Add SSLOptions to enable Diffie-Hellman key exchange and disable TLS/1.0.

Add SSLOptions to enable Diffie-Hellman key exchange and disable TLS/1.0.

DH/DHE support is being made optional because of known security issues with short
DH parameters.  Since there is no way to conditionally use DH/DHE with a minimum
number of bits, we just have to disable it by default.

TLS/1.0 support can now be disabled due to known security issues with TLS/1.0.
However, since TLS/1.1 and TLS/1.2 support is not universally available, we
cannot simply disable TLS/1.0 like we did for SSL/3.0.

git-svn-id: svn+ssh://src.apple.com/svn/cups/cups.org/trunk@12645 a1ca3aef-8c08-0410-bb20-df032aa958be