RFR: JDK-6996366 : convert MacAlg to an enum

[Jamil Nimeh]

Hello all,

This bug moves the internal MacAlg concrete class to an enum, and alters 
the CipherSuite constructor to no longer use String parsing on the 
cipher suite name to determine the MacAlg.  Instead, the constructor now 
requires the caller to pass in a MacAlg, similar to how it already takes 
a KeyExchange and BulkCipher.

Bug: https://bugs.openjdk.java.net/browse/JDK-6996366
Webrev: http://cr.openjdk.java.net/~jnimeh/reviews/6996366/webrev.01/

Thanks,
--Jamil