FIX: Correctly handle Windows paths that do not start with a drive letter

On Windows, an absolute path should start with a drive letter, or UNC notation.
`xolox#misc#path#split()` doesn't consider drive-local filespecs (e.g. `\Windows\system32`); it needs a check for a leading path separator like the Unix branch.
`xolox#misc#path#absolute()` doesn't convert a drive-local filespec into a full absolute one (e.g. `C:\Windows\system32`); it needs a check for those and prepend the current drive letter then.