RFR: JDK-8247734: [macos] jpackage fails to retrieve signing certificate when there are multiple

[Andy Herrick]

Please review fix for jpackage bug [1] at [2].

The behavior is to allow (after a warning message) case where user has 
multiple certificates matching the given mac-signing-key-user-name, and 
to allow the full certificate name to be used if it is given as user 
name (providing it matches expected certificate names)

/Andy

[1] - https://bugs.openjdk.java.net/browse/JDK-8247734

[2] - http://cr.openjdk.java.net/~herrick/8247734/webrev.01/index.html