Avoid using System.getProperty("line.separator") in messaging code

[ngmr]

Use of System.getProperty("line.separator") can run into trouble if Java 2 Security is enabled, as it induces a security check on the method call.

As minimum level of Java support is now (at least) Java 7, these calls can be safely replaced with calls to System.lineSeparator(), which does not induce a security check.