stddoclet: On overview page, break long package names onto a new line

Package names can get very long, making the first column of the
overview page very wide.

A reasonable solution is to break the package name onto a separate line
if the fully-qualified name exceeds 30 characters. Break the line by
inserting a normal space (not  ) after the dot. This is the simplest
fix, and will cause it to wrap to the next line. (The line will always
break even if the browser window is made full-width.) See:

http://webwork.eng/products/jdk/javadoc/trial/overview-summary-breakpackage-space.html