Fix Windows jvm.config parsing and add debug logging to scripts

- mvn.cmd: Use 'cmd /c' to run JvmConfigParser in a subshell, ensuring
  file handles are properly released before reading the temp file.
  The previous 'for /f' with backticks approach did not correctly
  capture the Java output on Windows.

- mvn, mvn.cmd: Add debug logging controlled by MAVEN_DEBUG_SCRIPT
  environment variable. When set, both scripts will output detailed
  information about jvm.config parsing, including:
  - Path to jvm.config file
  - Java command and arguments
  - Parser exit code and output
  - Final MAVEN_OPTS/JVM_CONFIG_MAVEN_OPTS value
  - Full JVM command line before launching Maven

This helps diagnose jvm.config issues on CI systems where the log
files are the primary debugging tool.

Author: gnodet

apache-maven/src/assembly/maven/bin/mvn

@@ -294,4 +294,9 @@ for arg in "$@"; do
     cmd="$cmd \"$arg\""
 done
 
+if [ -n "$MAVEN_DEBUG_SCRIPT" ]; then
+  echo "[DEBUG] Launching JVM with command:" >&2
+  echo "[DEBUG]   $cmd" >&2
+fi
+
 eval exec "$cmd"

apache-maven/src/assembly/maven/bin/mvn.cmd

@@ -272,6 +272,11 @@ for %%i in ("%MAVEN_HOME%"\boot\plexus-classworlds-*) do set LAUNCHER_JAR="%%i"
 set LAUNCHER_CLASS=org.codehaus.plexus.classworlds.launcher.Launcher
 if "%MAVEN_MAIN_CLASS%"=="" @set MAVEN_MAIN_CLASS=org.apache.maven.cling.MavenCling
 
+if defined MAVEN_DEBUG_SCRIPT (
+  echo [DEBUG] Launching JVM with command:
+  echo [DEBUG]   "%JAVACMD%" %INTERNAL_MAVEN_OPTS% %MAVEN_OPTS% %JVM_CONFIG_MAVEN_OPTS% %MAVEN_DEBUG_OPTS% --enable-native-access=ALL-UNNAMED -classpath %LAUNCHER_JAR% "-Dclassworlds.conf=%CLASSWORLDS_CONF%" "-Dmaven.home=%MAVEN_HOME%" "-Dmaven.mainClass=%MAVEN_MAIN_CLASS%" "-Dlibrary.jline.path=%MAVEN_HOME%\lib\jline-native" "-Dmaven.multiModuleProjectDirectory=%MAVEN_PROJECTBASEDIR%" %LAUNCHER_CLASS% %MAVEN_ARGS% %*
+)
+
 "%JAVACMD%" ^
   %INTERNAL_MAVEN_OPTS% ^
   %MAVEN_OPTS% ^