RFR: 8231552: ZGC: Refine address space reservation

[Per Liden]

We could be slightly more sophisticated and do a better job reserving 
address space in situations where parts of the address space is already 
occupied or when the process is running with address space limitations.

Bug: https://bugs.openjdk.java.net/browse/JDK-8231552
Webrev: http://cr.openjdk.java.net/~pliden/8231552/webrev.0

/Per