Simple patch to fix building debian package

[Mandy Chung]

jpkg fails to create the debian package for the JDK modules
because it uses '_' which is no longer a valid character
in the debian version.

This patch simply fixes to return the jigsaw version and
also checks for valid characters and throws exception if
the jigsaw version was changed to use any invalid character.
In addition, only include the mandatory dependencies in
the Depend statements for the debian package.

Webrev:
    http://cr.openjdk.java.net/~mchung/jigsaw/webrevs/jigsaw-deb-pkgs/

Mandy