asdf-system-directory on SBCL+Windows loses pathname device

[Anton Kovalenko]

SWANK-ASDF contrib contains a definition of asdf-system-directory, which is used for retrieving CL namestring of a system location for SLIME-BROWSE-SYSTEM.

PATHNAME-DEVICE is never passed to SLIME, causing browse-system to fail on SBCL+Windows (and surely on some other implementations, and maybe on some other platforms), unless a "device-less" pathname happens to be repaired from *DEFAULT-PATHNAME-DEFAULTS* (in order to reproduce, make sure that *DEFAULT-PATHNAME-DEFAULTS* doesn't have a PATHNAME-DEVICE similar to ASDF pathname).

Patch that works for me is attached. It is intended to be portable (and useful for any implementation where PATHNAME-DEVICE is important). I recommend to test it more broadly before committing (e.g. I didn't try CLISP, which is a usual suspect for pathname idiosyncrasies, and LW, and Allegro).