RFR (S): 8195817: JFR.stop should require name of recording

[Erik Gahlin]

Hi,

Could I please have a review of this fix.

Problem occurs if a user runs the following command:

 > jcmd <pid> JFR.stop

without specifying which recording to stop. The jcmd fails, but the 
output is "command executed successfully". Fix is to make the name 
parameter mandatory.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8195817

Webrev:
  http://cr.openjdk.java.net/~egahlin/8195817.1/

Testing:
Tests in test/jdk/jdk/jfr

Thanks
Erik