ECC Updates

A few updates/fixes to the ECC support added by 6405536 are needed:

 . add support for SHA256withECDSA (and 384/512) in addition to SHA1withECDSA

 . change the default key size/curve in keytool, EC KeyPairGenerator, and the SunJSSE ECDHE key exchange from NIST-P192 to NIST-P256. That is equivalent to 3072 bit RSA keys, so rather out of whack with our 1024 bit default for RSA, but NSA Suite B mandates P256 and it it also more widely implemented than P192.

 . the P11KeyStore does not really understand EC keys, so it is not possible to store them into a PKCS#11 token. This needs to be fixed, maybe along with some special code for some preexisting NSS specific problems.
Also:

 . the "Supported Elliptic Curves Extension" is encoded incorrectly. This causes problems if a JSSE client is talking to an ECC server that parses this extension. By accident, JSSE in server mode is ok.