RFR JDK-8239950: Update PKCS9 Attributes to PKCS#9 v2.0 Encodings

[Jamil Nimeh]

Hello all,

This brings a few PKCS#9 attributes (unstructuredName, 
unstructuredAddress, signingTime) into line with v2.0 of the spec (RFC 
2985).  It mostly expands the allowed string or date types for these 
attributes.  I also came across a corner-case bug where toString calls 
on PKCS9Attribute objects were throwing NPE if the attribute type was a 
UniversalString, so that is addressed in this webrev too.

Webrev: https://cr.openjdk.java.net/~jnimeh/reviews/8239950/webrev.01

JBS: https://bugs.openjdk.java.net/browse/JDK-8239950

--Jamil