Use prefdir() as storage location for javaclasspath.

At least on unix(), this is the documented location for this file in R2013b.
Fix check for empty UserInfo. getUserInfo returns empty double if no value is found, but xyz.isEmpty does not work for MATLAB double class (in R2013b).
Fix printing user and email info - use explicit char() conversion.
Informational: print path of created javaclasspath.txt.