RFR 9: 8178347 Process and ProcessHandle getPid method name inconsistency

[Roger Riggs]

Please review this change to the name of the ProcessHandle.getPid and 
Process.getPid methods
as observed and recommended in email: [1]

The methods are new in JDK 9 and the renaming corrects the method name 
style to be
consistent within each class.

The change sets propagate the name change to the openjdk uses of the pid 
method.
Libraries using getPid will need to be updated.

Issue:
  https://bugs.openjdk.java.net/browse/JDK-8178347

Webrevs:

http://cr.openjdk.java.net/~rriggs/webrev-pid-8178347
http://cr.openjdk.java.net/~rriggs/webrev-pid-jaxp-8178347
http://cr.openjdk.java.net/~rriggs/webrev-pid-jdk-8178347
http://cr.openjdk.java.net/~rriggs/webrev-pid-hotspot-8178347

Thanks, Roger

[1] 
http://mail.openjdk.java.net/pipermail/core-libs-dev/2017-April/047103.html