RFR 9: 8071481: (Process) Merge UNIXProcess.java into ProcessImpl.java

[Roger Riggs]

Please review this merge of UNIXProcess.java with ProcessImpl.java.

For Unix variants of Process, the implementation is split between
src/java.base/unix/classes/java/lang/ProcessImpl.java and UNIXProcess.java.
ProcessImpl.java contains only static methods and delegates to 
UNIXProcess.java.
Merging UNIXProcess.java into ProcessImpl.java removes a redundant method
and aligns the source files with the Windows implementation that 
contains only ProcessImpl.java.

Webrev:
  http://cr.openjdk.java.net/~rriggs/webrev-unixprocess-rename-8071481/

Issue:
   8071481: (Process) Merge UNIXProcess.java into ProcessImpl.java

[1] https://bugs.openjdk.java.net/browse/JDK-8071481