muffin cache should be created lazily

The muffin directory seems to be created all the time
even though it is not used. For example, run the
installer.sh on Solaris and Linux, then it will create
 
 ~/.javaws/muffins

This might be undesirable since it could be created by
the wrong user and permissions if the installation was
run with "su root". We should create this lazily on first
use.