javadoc -encoding doesn't work when using the old doclet API

In JDK 9, the javadoc flag -encoding fails to override the default encoding when using the old doclet API.

The following patch fixes the bug:

```
diff --git a/src/jdk.javadoc/share/classes/com/sun/tools/javadoc/main/ToolOption.java b/src/jdk.javadoc/share/classes/com/sun/tools/javadoc/main/ToolOption.java
--- a/src/jdk.javadoc/share/classes/com/sun/tools/javadoc/main/ToolOption.java
+++ b/src/jdk.javadoc/share/classes/com/sun/tools/javadoc/main/ToolOption.java
@@ -157,10 +157,11 @@
     ENCODING("-encoding", true) {
         @Override
         public void process(Helper helper, String arg) {
             helper.encoding = arg;
             helper.setCompilerOpt(opt, arg);
+ helper.setFileManagerOpt(Option.ENCODING, arg);
         }
     },
 
     RELEASE("--release", true) {
         @Override
```

Repro:

=== Doclet.java
import com.sun.javadoc.*;
public class Doclet extends com.sun.javadoc.Doclet {
  public static LanguageVersion languageVersion() {
    return LanguageVersion.JAVA_1_5;
  }
}
=== Test.java
/** 🥑 */
class Test {}
===

$ javac -cp $JAVA_HOME/lib/tools.jar Doclet.java

with 1.8.0_152-ea:

$ javadoc -J-Dfile.encoding=ascii -doclet Doclet -docletpath . -encoding UTF-8 Test.java
... OK

with 9+181:

$ javadoc -J-Dfile.encoding=ascii -doclet Doclet -docletpath . -encoding UTF-8 Test.java
javadoc: warning - The old Doclet and Taglet APIs in the packages
com.sun.javadoc, com.sun.tools.doclets and their implementations
are planned to be removed in a future JDK release. These
components have been superseded by the new APIs in jdk.javadoc.doclet.
Users are strongly recommended to migrate to the new APIs.
Loading source file Test.java...
Test.java:1: error: unmappable character (0xF0) for encoding US-ASCII
/** ???? */
    ^