[#391] Use System.lineSeparator() instead of System.getProperty(\"lin…

[Neil-Tomar]

…e.separator")

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[Neil-Tomar]

Fixes #391

Replaces System.getProperty("line.separator") with System.lineSeparator(),
which cannot return null and has been available since Java 7.

• AbstractHelpMojo.LS
• DescribeMojoTest (which previously mixed both forms)
• Updates the src/it verify.groovy scripts that used the old form
  (per discussion on the issue).