base, optparse: Various option-parsing enhancements.

base, optparse: Various option-parsing enhancements.

  * `case2' clauses can optionally bind a variable to its scrutinee.

  * New function `option-parse-return' to return a value from
    `option-parse-next'.

  * Enhance `options' parse-option-form' to understand disambiguating
    keywords arguments for all option slots.  This also means that these
    things can be set from expressions rather than constants.

  * Default the `define-program' variables sensibly.

  * Make `do-options' use `let*/gensyms'.

  * Make the optparse test use the convenience macros.

  * Rename `help-opts' to `help-options'.  Just because.  And tidy up
    the code a bunch.