RFR: JDK-8232935: jpackage failed with NPE whenever --file-associations provided

[Alexey Semenyuk]

Please review fix [2] for jpackage bug [1].

Move function checking values of mime types in file associations from 
Linux to shared code. Added jtreg tests to cover use cases when no or 
multiple mime types are specified for file associations.

- Alexey

[1] https://bugs.openjdk.java.net/browse/JDK-8232935

[2] http://cr.openjdk.java.net/~asemenyuk/8232935/webrev.00