RFR(s): 8204958: Minor cleanups for the diagnostic framework

[Thomas Stüfe]

Hi all,

while working on jcmd I saw some minor cleanup possibilities.

Webrev: http://cr.openjdk.java.net/~stuefe/webrevs/8204958-jcmd-cleanups/webrev.00/webrev/
Bug: https://bugs.openjdk.java.net/browse/JDK-8204958

Most of them are const correctness and using initializer lists instead
of java-style member initialization in these C++ classes.

Other changes:

- Added a ResourceMark into the DCmd::parse_and_execute() loop
- removed DCmdFactory::create_global_DCmd() (and following that,
DCmdFactory::create_Cheap_instance()) since I did not find anyone
using that function.
- Also in DCmdFactory, I removed a number of setters which were never
called and for attributes which should be immutable: hidden, flags,
enabled.

This is really a rather light cleanup. It did not touch any larger issues.

Thank you,

Thomas