The definition for each OPTIONS= option resides in include/optlist.h as of
February 2020 in the WIP branch.

Boolean and compound options are combined into a single allopt[] array.

To add an entirely new option macro type:

    The current list of option macros:
    NHOPTB (for boolean options)
    NHOPTC (for complex options that take values beyond off/on)
    NHOPTP (for handling a suite that all begin with a common prefix)
    NHOPTO (for other options not easily handled as one of the above)

     1. You need to add three different expansions of your macro, one for
        the NHOPT_PROTO phase where function prototypes are generated,
        one for the NHOPT_ENUM phase where enums are generated, and
        NHOPT_PARSE phase.

     2. If you are adding a new macro type to the current NHOPTB,
        NHOPTC, NHOPTP, NHOPTO macros, don't forget to add the #undef near
        the bottom of optlist.h, so that the macro is available for re-use
        during the next usage phase.

 To add an option:

     1. Add an entry to include/optlist.h, using the NHOPTB macro for a
       boolean on/off option, or NHOPTC macro for a complex option,
       or NHOPTP macro if it handles an entire prefix, or NHOPTO macro for
       other types.

       The list of options is always kept in optlist.h in alphabetical order.

       When the list of options is processed during the compile of options.c,
       the following will be automatically generated and included in
       options.c:

            i) an optfn_xxxx function prototype (xxxx is the option name).
           ii) an opt_xxxx enum value for referencing that option index by
               name throughout options.c (xxxx is the option name).
          iii) an initialization of an element in the allopt[] array, at
               index opt_xxxx from step ii (xxxx is the option name).

    2. If you are adding a boolean option, link it to a global variable
       (e.g. in flags or iflags) and update optfn_boolean in options.c if
       necessary.

    3. If you are not adding a boolean option, create the optfn_xxxx()
       function in options.c. Failure to do that will result in a link error
       of "undefined function optfn_xxxx." The functions are in options.c in
       alphabetical sequence by function name.

       The skeletal template for an optn_xxxx() function is:

            int
            optfn_xxxx(optidx, req, negated, opts, op)
            int optidx              /* the index of this option opt_xxxx */
            int req;                /* the request ID from core functions */
            boolean negated;        /* will be true if opt was negated */
            char *opts;             /* points to the complete opt string */
            char *op;               /* points to value portion of opt string */
            {
                if (req == do_init) {
                    return optn_ok;
                }
                if (req == do_set) {
                    /* do option set processing for the option */
                    /* if successful, return optn_ok;    */
                    /* if unsuccessful, return optn_err; */
                }
                if (req == get_val) {
                    /* return the current val of option in supplied opts buf */
                    if (!opts)
                        return optn_err;
                    Sprintf(opts, "%s", fakefunction_to_get_xxxx_value_goes_here);
                    return optn_ok;
                }
                if (req == do_handler) {

                    /* this is optional. If the option needs its own
                       special handling after the doset menu, do it here
                       or call a function to do it. The naming convention
                       for such a function in options.c has been
                       handler_xxxx(), but the function does not need to
                       reside in options.c */

                    return optn_ok;
                }
                return optn_ok;
            }

    4. NOTE:  If you add (or delete) an option, please update the short
       options help (option_help()), the long options help (dat/opthelp)
       and also the Guidebooks.

Here's some information about the req ID's passed to optfn_xxxx() functions.
Each optfn_xxxx() function can be called with a req id of: do_init, do_set,
do_handler or get_val.

req do_init is called from options_init, and if initialization or memory
allocation or other initialization for that particular option is needed,
it can be done in response to the init req

req do_set is called from parseoptions() for each option it encounters
and the optfn_xxxx() function is expected to react and set the option
based on the string values that parseoptions() passes to it.

req get_val is passed a buffer from its caller that the optfn_xxxx() is
expected to fill with the current value of the option.

req do_handler is called during doset() operations processing in response
to player selections, most likely from the 'O' option-setting menu. The
do_handler req is only called for options that were marked as supporting
do_handler in the option definition in include/optlist.h
