RFR: JDK-8231721: jpackage --install-dir should reject system dirs on Linux

[Alexey Semenyuk]

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

This is a fix for the JDK-8200758-branch branch of the open sandbox 
repository (jpackage).

[1] https://bugs.openjdk.java.net/browse/JDK-8231721

[2] http://cr.openjdk.java.net/~asemenyuk/8231721/webrev.00/