RFR: JDK-6996366 : convert MacAlg to an enum

[Wang Weijun]

One disadvantage of enum to constant is that third-party library will not be able to extend it. I noticed this when I was trying to extend TLS to use non-certificate-based ciphersuites by providing more KeyExchange values. That work is stalled now but if we want to do it later this should be considered. Can MacAlg also be extended?

--Max

>> 
>> http://cr.openjdk.java.net/~jnimeh/reviews/6996366/webrev.02/
>>