RFR 8067447: Factor out the shared implementation of the VM flags manipulation code

[Jaroslav Bachorik]

Hi, this is a round 2 review of the proposed solution.
Issue : https://bugs.openjdk.java.net/browse/JDK-8067447
Webrev: http://cr.openjdk.java.net/~jbachorik/8067447/webrev.02

This patch is a precursor for implementing 
https://bugs.openjdk.java.net/browse/JDK-8054890 which itself is a part 
of JEP-228 (https://bugs.openjdk.java.net/browse/JDK-8043764).

Here, the code related to manipulating JVM flags is extracted to a 
separate WritableFlags class and the codebease is adjusted to use this 
class.

I didn't touch the original (nonstandard) handling of the DTrace 
specific flags in the Solaris specific attachListener.cpp implementation 
to keep the change simple and focused.

Thanks,

-JB-