RFR: Improved, more flexible heap region size settings

[Roman Kennke]

Currently, the heap region size is determined using hardcoded min (1M),
max (32M) and target number (2048). We also have an option to
explicitely set regions size, but it will still not go below or over
the hardcoded bounds.

This change turns the 3 hardcoded values into proper cmd line args, and
also changes -XX:ShenandoahHeapRegionSize to override whatever bounds
are set (default or not). All 3 arguments are checked for sanity. I'm
including a jtreg test for the args checking.

http://cr.openjdk.java.net/~rkennke/heapregionsize/webrev.00/

Ok?

Roman