Remove convoluted keyword argument processing in RESTART-CASE

Remove convoluted keyword argument processing in RESTART-CASE

* Replace a combination of TRANSFORM-KEYWORDS, PARSE-KEYWORD-PAIR and
  WITH-KEYWORD-PAIRS with a relatively simple local function
  PARSE-KEYWORDS-AND-BODY; Gets rid of KLUDGE regarding
  WITH-KEYWORD-PAIRS

* Add a smoke test and more cases for the :MALFORMED-CLAUSES test in
  tests/condition.pure.lisp; Gets rid of the "fair amount of
  rearrangement ... should be tested" FIXME

* Fix "test case from Gerd Moellmann" in tests/clos.impure.lisp which
  contained an invalid RESTART-CASE form uncovered by the above change