[RFR]8215622: Add dump to file support for jmap histo

[臧琳]

Hi All,
       May I ask your help to review this patch for enhance jmap �Chisto.
It add the “file=<path>” arguments that allow jmap �Chisto outputs data to file 
directly.
This patch is also part of the enhancement described in 
https://bugs.openjdk.java.net/browse/JDK-8214535.

Webrev: http://cr.openjdk.java.net/~xiaofeya/8215622/webrev.00/
Bug: https://bugs.openjdk.java.net/browse/JDK-8215622

Thanks.

BRs,
Lin