Implement Process#groups for non-Windows machines

[cheald]

Implements Process#groups. This relies on the com.sun.security.auth.module.UnixSystem class, which is only present in *nix distributions of the JRE; it is actually not present on Windows (a similar NTSystem module is instead present).

I've attempted to build JRuby on Windows, but my Windows build environment is sadly not up to date. However, based on some Googling, avoiding the import and instead referencing the full package name at runtime should allow the project to compile on Windows while still avoiding referencing classes that don't exist at runtime.

[headius]

This may or may not work to build on Windows, but we will have to cross the "building on Windows" bridge sooner rather than later. If it turns out this doesn't work, we can turn it into full reflection or use some other mechanism (mock library) to provide the functionality.

[headius]

Thank you!