[MCOMPILER-572] Multi-line <arg> does not work on Windows when forked

**[Mikkel Kjeldsen](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=JIRAUSER303566)** opened **[MCOMPILER-572](https://issues.apache.org/jira/browse/MCOMPILER-572?redirect=false)** and commented

Versions of `maven-compiler-plugin` that rely on `plexus-compiler-2.14.2`, hereunder `maven-compiler-plugin-3.12.1`, cannot pass multi-line compiler arguments to a forked `javac` on Windows in any way. Further, attempting to do so produces a non-obvious and difficult to troubleshoot error. This error does not manifest outside of Windows, nor does it manifest on Windows if the compiler is not forked.

This bug is a downstream version of https://github.com/codehaus-plexus/plexus-compiler/issues/351, filed in response to https://github.com/google/error-prone/issues/4256.

The sample POM

```xml

4.0.0
example.multi-line-fork-crash
minimally-reproducible
1.0-SNAPSHOT

21
3.12.1




org.apache.maven.plugins
maven-compiler-plugin
${maven-compiler-plugin.version}

true


-Xplugin:ErrorProne \
-Xep:DeadException:WARN

```

_should_ fail to compile with the error

```
plug-in not found: ErrorProne
```

but instead fails with the error

```
error: invalid flag: -Xep:DeadException
```

because `plexus-compiler` indiscriminately translates the \ to / and breaks `javac` 's multi-line argument support.

---

**Affects:** 3.12.1

**Issue Links:**
- [MCOMPILER-425](https://issues.apache.org/jira/browse/MCOMPILER-425) Compiler argument with newline treated as multiple arguments in fork mode

**Remote Links:**
- [Maven multi-line \ does not work on Windows when \ed
](https://github.com/google/error-prone/issues/4256)
- [Forking JavacCompiler breaks multiline arguments on Windows
](https://github.com/codehaus-plexus/plexus-compiler/issues/351)