RFR: 8327410: Add hostname option for UL file names

[Fredrik Bredberg]

Added %hn as way to make the Unified Logging file name include the host name.
This is done in line with the already existing %p for pid, and %t for time stamp.

Also the %hn decorator is already used to prepend the host name to each log line. See: The [JDK-8148219](https://bugs.openjdk.org/browse/JDK-8148219).

Example: `java -Xlog:gc+init:file=frbr_%hn.%p.%t.log -version`

Tested tier1-tier5.

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Commit messages:
 - 8327410: Add hostname option for UL file names

Changes: https://git.openjdk.org/jdk/pull/18153/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18153&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8327410
  Stats: 91 lines in 2 files changed: 35 ins; 24 del; 32 mod
  Patch: https://git.openjdk.org/jdk/pull/18153.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18153/head:pull/18153

PR: https://git.openjdk.org/jdk/pull/18153