RFR 8197387: jcmd started by "root" must be allowed to access all VM processes

[Daniil Titov]

Please review the changes that fix JDK-8197387.

There are 2 problems here:
1. JVM ignores  .attach_pid<pid> file if it is owned by the user different from the one that owns this JVM process 
2. jcmd checks that .java_pid<pid> socket is owned by the same user that runs jcmd and reports an error otherwise  

The fix relaxes these checks to allow jcmd started by  "root"  (UID = 0) access JVMs started by another users.

Bug: https://bugs.openjdk.java.net/browse/JDK-8197387
Webrev: http://cr.openjdk.java.net/~dtitov/8197387/webrev.01/

Best regards,
Daniil