[MJLINK-73] Add fork mode

**[Benjamin Marwell](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=mampf86)** opened **[MJLINK-73](https://issues.apache.org/jira/browse/MJLINK-73?redirect=false)** and commented

Instead of just using Toolchains, we could add a fork=true parameter which will do the same but with the jlink executable from JAVA_HOME.
Hint: Due to toolchains, this is actually the default.

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**Remote Links:**
- [GitHub Pull Request #282
](https://github.com/apache/maven-jlink-plugin/issues/282)
- [GitHub Pull Request #313
](https://github.com/apache/maven-jlink-plugin/issues/313)
- [GitHub Pull Request #378
](https://github.com/apache/maven-jlink-plugin/issues/378)
- [GitHub Pull Request #422
](https://github.com/apache/maven-jlink-plugin/issues/422)
- [GitHub Pull Request #473
](https://github.com/apache/maven-jlink-plugin/issues/473)

No source files or tests are named. Start by inspecting the Maven plugin's existing Toolchains and jlink configuration handling, then trace how executable selection is tested. Done means a fork=true parameter is supported and uses the jlink executable from JAVA_HOME as described, with behavior aligned to the existing toolchain default.

Written by the indexing model from the issue text.