path/filepath, os/exec: unquote PATH elements on Windows

On Windows, directory names in PATH can be fully or partially quoted
in double quotes ('"'), but the path names as used by most APIs must
be unquoted. In addition, quoted names can contain the semicolon
(';') character, which is otherwise used as ListSeparator.

This CL changes SplitList in path/filepath and LookPath in os/exec
to only	treat unquoted semicolons as separators, and to unquote the
separated elements.

(In addition, fix harmless test bug I introduced for LookPath on Unix.)

Related discussion thread:
https://groups.google.com/d/msg/golang-nuts/PXCr10DsRb4/sawZBM7scYgJ

R=rsc, minux.ma, mccoyst, alex.brainman, iant
CC=golang-dev
https://codereview.appspot.com/7181047

Committer: Alex Brainman <alex.brainman@gmail.com>