Define SSL_AD_* as actual values.

Define SSL_AD_* as actual values.

Rather than having SSL_AD_* as defines that refer to SSL3_AD_* or
TLS1_AD_*, just give them actual values directly since it is more readable
and the indirection provides no value. Place SSL3_AD_* and TLS1_AD_* under
#ifndef LIBRESSL_INTERNAL to prevent further usage.

ok tb@