[SUREFIRE-2184] debugForkedProcess should implicitly set forkCount to 1

**[Konrad Windszus](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=kwin)** opened **[SUREFIRE-2184](https://issues.apache.org/jira/browse/SUREFIRE-2184?redirect=false)** and commented

Very often the parameter [`debugForkedProcess`](https://maven.apache.org/surefire/maven-surefire-plugin/test-mojo.html#debugForkedProcess) is set through Maven CLI. When used in combination with [`forkCount`](https://maven.apache.org/surefire/maven-surefire-plugin/test-mojo.html#forkcount) > 1 this always leads to issues as the debug port is obviously blocked if it is already taken for one fork. Therefore it would be good to implicitly set the `forkCount` to 1 in case `debugForkedProcess` is set (with a WARNING in the log)

---

**Affects:** 3.1.2

Start by tracing how the Surefire plugin handles the debugForkedProcess and forkCount parameters, including values supplied through the Maven CLI. Confirm the behavior around multiple forks and the debug port, then verify that enabling debugForkedProcess uses one fork and emits a warning.

Written by the indexing model from the issue text.