RFR(s): 8067768: Check for too small values for -Xmx

[Per Liden]

Hi,

Summary: The VM needs at least a 2M heap (MaxHeapSize) and at least 1M 
initial heap (InitialHeapSize), but we only check the InitialHeapSize 
limit during start up. This patch adds a check for MaxHeapSize and 
extends the jtreg heap size test to check these limits.

Bug: https://bugs.openjdk.java.net/browse/JDK-8067768
Webrev: http://cr.openjdk.java.net/~pliden/8067768/webrev.0/
Testing: manual, jtreg test/gc/*, jprt in progress

thanks,
Per