javadoc does not (over)write stylesheet.css

javadoc does not write stylesheet.css if such a file already exists in the output directory.

This means it does not get updated correctly if it is different from the version that would be written by javadoc.

$ more $(find play/stylesheet/src/ -name \*.java) play/stylesheet/run.sh
::::::::::::::
play/stylesheet/src/p/Test.java
::::::::::::::
package p;

/**
 * This is a test.
 */
public class Test { }
::::::::::::::
play/stylesheet/run.sh
::::::::::::::
#!/bin/sh

jdk=build/*/images/jdk

mkdir -p play/stylesheet/api
echo "BAD" > play/stylesheet/api/stylesheet.css
$jdk/bin/javadoc -d play/stylesheet/api -sourcepath play/stylesheet/src p
grep "BAD" play/stylesheet/api/stylesheet.css

$ sh play/stylesheet/run.sh
Loading source files for package p...
Constructing Javadoc information...
javadoc: warning - You have not specified the version of HTML to use.
The default is currently HTML 4.01, but this will change to HTML5
in a future release. To suppress this warning, please specify the
version of HTML used in your documentation comments and to be
generated by this doclet, using the -html4 or -html5 options.
Standard Doclet version 11-internal
Building tree for all the packages and classes...
Generating play/stylesheet/api/p/Test.html...
Generating play/stylesheet/api/p/package-frame.html...
Generating play/stylesheet/api/p/package-summary.html...
Generating play/stylesheet/api/p/package-tree.html...
Generating play/stylesheet/api/constant-values.html...
Building index for all the packages and classes...
Generating play/stylesheet/api/overview-tree.html...
Generating play/stylesheet/api/index-all.html...
Generating play/stylesheet/api/deprecated-list.html...
Building index for all classes...
Generating play/stylesheet/api/allclasses-frame.html...
Generating play/stylesheet/api/allclasses-frame.html...
Generating play/stylesheet/api/allclasses-noframe.html...
Generating play/stylesheet/api/allclasses-noframe.html...
Generating play/stylesheet/api/index.html...
Generating play/stylesheet/api/help-doc.html...
1 warning
BAD