absolute paths without drive letter don't work in windows

As originally reported

https://jira.codehaus.org/browse/JRUBY-5750

assuming you have an empty file in c:\dev\yo

> > Dir.chdir("c:\tmp") # works
> > File.read('/dev/yo')
> > Errno::ENOENT: No such file or directory - C:/dev/dev/yo
> > ...
> > File.exist?('/dev/yo')
> > => false # should be true
