Downgrade normative references to ${java.home}/lib/security/krb5.conf

This is motivated by JEP 162, to ease the transition to modules in JDK 9. The relevant section of http://openjdk.java.net/jeps/162 is:

Review normative specification references to files in $JAVA_HOME and where possible downgrade these references to non-normative status. By downgrading these references then it will allow their location (or format) to be changed in a future release. For example the property file for currencies could be moved into a module-private location.

The reference in this case occurs in the package description of javax.security.auth.kerberos:

"You can provide the name of your default realm and Key Distribution Center (KDC) host for that realm using the system properties java.security.krb5.realm and java.security.krb5.kdc. Alternatively, you can provide an MIT style configuration file called krb5.conf in <java-home>/lib/security. If you place this file elsewhere, you can indicate that location via the system property java.security.krb5.conf."