JDK 9 RFR of JDK-8035279: Clean up internal deprecations in BigInteger

[Brian Burkhalter]

This patch concerns cleaning up some ugly internal deprecations.

Issue:	https://bugs.openjdk.java.net/browse/JDK-8035279
Webrev:	http://cr.openjdk.java.net/~bpb/8035279/webrev.00/

All JTREG BigInteger tests pass, and the serialized form has been unaltered as verified by bidirectional read-write testing between Java 7 and this version.

I would appreciate scrutiny of the arithmetic to make sure that I've made no silly errors. Also I would be interested in opinions as to whether the "volatile" keyword should in fact be used for the affected instance variables.

Thank you for your comments.

Brian