[MJAVADOC-650] Fix javadoc binary search for JDK 9+.

Imported patch from Debian. In JDK 9 the jre directory was removed from
the JDK so the lookup path should be modified.

https://sources.debian.org/data/main/m/maven-javadoc-plugin/3.0.1-3/debian/patches/openjdk-9-javadoc-path.patch

Author: Chris West

src/main/java/org/apache/maven/plugins/javadoc/AbstractJavadocMojo.java

@@ -3799,6 +3799,12 @@ else if ( SystemUtils.IS_OS_MAC_OSX && !JavaVersion.JAVA_SPECIFICATION_VERSION.i
         {
             javadocExe = new File( SystemUtils.getJavaHome() + File.separator + "bin", javadocCommand );
         }
+        // On Java 9, this has moved to: /usr/lib/jvm/java-9-openjdk-amd64/bin/javadoc
+        else if ( org.apache.commons.lang3.SystemUtils.isJavaVersionAtLeast( org.apache.commons.lang3.JavaVersion.JAVA_9 ) )
+        {
+            javadocExe =
+                new File( SystemUtils.getJavaHome() + File.separator + "bin", javadocCommand );
+        }
         else
         {
             javadocExe =