RFR 9: 8149750 Decouple sun.misc.Signal from the base module

[Roger Riggs]

Please review moving the functionality of sun.misc.Signal to 
jdk.internal.misc and
creating a wrapper sun.misc.Signal for existing external uses.
A new replacement API will be considered separately.

The update includes a test that passes with or without the changes.
(Except for an NPE instead of SEGV if null is passed).

Webrev:
    http://cr.openjdk.java.net/~rriggs/webrev-signal-9149750/

Issue:
   https://bugs.openjdk.java.net/browse/JDK-8149750

Thanks, Roger

JEP 260: https://bugs.openjdk.java.net/browse/JDK-8132928