RFR: 8327410: Add hostname option for UL file names [v2]

[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.

Fredrik Bredberg has updated the pull request incrementally with one additional commit since the last revision:

  Get the hostname only if the user explicitly requests it

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/18153/files
  - new: https://git.openjdk.org/jdk/pull/18153/files/8bb03b77..57a03469

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18153&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18153&range=00-01

  Stats: 17 lines in 3 files changed: 4 ins; 8 del; 5 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