RFR: (8031737) CHECK_NULL and CHECK_EXCEPTION macros cleanup

[roger riggs]

Hi,

A few small changes to cleanup the name and use of CHECK macros.
The CHECK_NULL_* macros are more generally useful without the JNU_ prefix.
The JNU_CHECK_EXCEPTION* macros apply only when there is an jni environment.

Webrev:
     http://cr.openjdk.java.net/~rriggs/webrev-check-cleanup-8031737/

Thanks, Roger

[1] https://bugs.openjdk.java.net/browse/JDK-8031737