parse-namestring ignores current directory spec

[mmontone]

(parse-namestring "./foo") evaluates to #p"foo", instead of #p"./foo".

Most Lisp implementation evaluate to #p"./foo", but I don't think this is in the standard.

ABCL supports current directory in paths though, using make-pathname:

(make-pathname :directory '(:relative ".") :name "foo") => #P"./foo".

So it is only parse-namestring that would need to be modified.

[easye]

Verified that other open implementations (sbcl, ecl, ccl) have this behavior.

Added test for subsequent patches of PARSE-NAMESTRING.