RFR 8215776: Keytool importkeystore may mix up certificate chain entries when DNs conflict

[Weijun Wang]

Please take a review at

  https://cr.openjdk.java.net/~weijun/8215776/webrev.00/

PKCS12KeyStore now can find certificate issuers more precisely using 
SubjectKeyIdentifier and AuthorityKeyIdentifier. I thought about using CertPath 
builder or checking signatures but those changes are too much.

Thanks,
Max