cvs-plugin part of fix for JENKINS-23234: jenkins cvs update hang when r...

ignore symlinks (when recursing) like the major cvs clients.
The javadoc doesn't need to specify what issue this method was introduced for, only that the method does and any use cases anyone need to be aware of. This could be simplified to:

> Check if the given file is a symbolic link. Useful for preventing CSV recursing into directories infinitely.
> @param file name of the file to test
> @return whether the file if believed to be a symlink or not