Experimental encrypt-then-mac support.

Experimental support for encrypt then mac from
draft-gutmann-tls-encrypt-then-mac-02.txt

To enable it set the appropriate extension number (0x10 for the test server)
using e.g. -DTLSEXT_TYPE_encrypt_then_mac=0x10

For non-compliant peers (i.e. just about everything) this should have no
effect.