File.exist? and many others do not try system dir on wnidows if path starts with bare /

[enebo]

In fixing #2291, I noticed that for most file operations we do not allow a bare / path (e.g. /opt vs C:/opt) to resolve as a proper absolute path. I put in a one-off fix for #2291, but ultimately this should be supported by our internal file support itself.

[enebo]

This I realize is too vague and we have not quite gotten appveyor running jruby test suites so I will close this and open individual issues when we get things running better on windows.