RFR(s): 8178364: Command-line flags of type double should accept integer values

[Per Liden]

Hi,

For convenience, command-line flags of type double should accept integer 
values (as well as the normal double values). This would allow users to 
write the short form, e.g:

-XX:G1ConcMarkStepDurationMillis=10

instead of always having to do:

-XX:G1ConcMarkStepDurationMillis=10.0

Bug: https://bugs.openjdk.java.net/browse/JDK-8178364
Webrev: http://cr.openjdk.java.net/~pliden/8178364/webrev.0/
Testing: JTreg test added, passes jprt

cheers,
Per