RFR 8054890: Serviceability: New diagnostic commands 'VM.set_flag' and 'JVMTI.data_dump'

[Jaroslav Bachorik]

Please, review the following change

Issue : https://bugs.openjdk.java.net/browse/JDK-8054890
Webrev: http://cr.openjdk.java.net/~jbachorik/8054890/webrev.00

This patch is about adding 2 new diagnostic commands - VM.set_flag and 
JVMTI.data_dump.

VM.set_flag allows to set any writeable flag. It takes the flag name and 
the flag value in textual form. The mutability of the flag and the value 
format checks are forwarded to the shared vm management code.

JVMTI.data_dump will send the data dump request to JVMTI.

Both of these commands are covered by the corresponding tests.

Thanks,

-JB-