RFR: 8261441: JFR: Filename expansion

[Denghui Dong]

Hi,
Could I have a review of this change that let the users could use `%p` in the filename to represent the PID  when JFR.start/stop/dump.

I haven't implemented `%t` described in the issue, because I don't think it's very useful.

Best,
Denghui Dong

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Commit messages:
 - 8261441: JFR: Filename expansion

Changes: https://git.openjdk.java.net/jdk/pull/4550/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4550&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261441
  Stats: 113 lines in 5 files changed: 105 ins; 0 del; 8 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4550.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4550/head:pull/4550

PR: https://git.openjdk.java.net/jdk/pull/4550