RFR: 8261482: Adjust jmap -histo to accept noparallel option to inspect heap serially

[Lin Zang]

8261482: Adjust jmap -histo to accept noparallel option to inspect heap serially

-------------

Commit messages:
 - 8261482: Adjust jmap -histo to accept noparallel option to inspect heap serially

Changes: https://git.openjdk.java.net/jdk/pull/2519/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2519&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261482
  Stats: 50 lines in 4 files changed: 7 ins; 22 del; 21 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2519.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2519/head:pull/2519

PR: https://git.openjdk.java.net/jdk/pull/2519