Assorted fixes pointed out by Fare

* swank-loader.lisp (*load-path*): Moved up to avoid forward
reference.

* swank-ecl.lisp: Add warning about uncommenting code that depends
on swank-loader.lisp.

* start-swank.lisp: Use MAKE-PATHNAME instead of MERGE-PATHNAMES.

* sbcl-pprint-patch.lisp: missing closing parenthesis.

* contrib/swank-presentation-streams.lisp [sbcl]: Don't rely on
  swank-loader.lisp.