Use of Unix security module breaks build on Windows

[chrisseaton]

Commit be98526 breaks the build on Windows, as I think com.sun.security.auth.module.UnixSystem is not available.

RubyProcess.java:[579,61] error: cannot find symbol

Solution is to use reflection to load the class.

Already discussed with @cheald - opening issue to track.

[enebo]

I think this should be moved into jnr-posix as the pure-Java version. Then we can remove any need for reflective soln from that library (which already calculates platform-specific info).

[enebo]

I still think we should move this to jnr-posix, but for now it makes sense to allow JRuby to compile on Windows again :)