isRelative
Is a path relative, or is it fixed to the root?
> Windows: isRelative "path\\test" == True
> Windows: isRelative "c:\\test" == False
> Windows: isRelative "c:test" == True
> Windows: isRelative "c:" == True
> Windows: isRelative "\\\\foo" == False
> Windows: isRelative "/foo" == True
> Posix: isRelative "test/path" == True
> Posix: isRelative "/test" == False
Test if string contains a valid relative URI (a relative URI with optional fragment identifier).