Current directory (lodaer.path=.) pathology workaround

It turns out that loader.path=. was pathological and before this
change ended up making the classpath empty (loader.path=.,lib/
would have fixed it). With this change the old behaviour is still
supported, but if the only user-supplied path entry is "." (or
empty) then it is now kept, and translates into the root of the
current archive if running as "java -jar ...".

Fixes gh-270