getopts: new feature: cumulative options.

An option declared as (cumul <type>) indicates that it is of
type <type>, and that multiple occurrences of the option
produce values that are accumulated into a list.
The accumulation is in reverse order: the rightmost
occurrence ends up the first in the list.

* lisplib.c (getopts_set_entries): Add cumul to list of
interned symbols, so that the getopts.tl code isn't mistakenly
working with sys:cumul.

* share/txr/stdlib/getopts.tl (opt-parsed): New slot, cumul.
(opt-desc basic-type p, opt-desc list-type-p,  opt-desc
cumul-type-p): New methods.
(opt-desc check): Rework type validity check using the new
methods.
(opt-parsed convert-type): Support 'cumul type by
instantiating an opt-parsed object for the wrapped type,
and stealing its converted argument into the current object,
and setting the cumul flag.
(opts add-opt, option-base add-opt): Support options that have
the cumul flag set by accumulating list values. The code is
different due to different amounts of encapsulation. The opts
structure stores the raw opt-parsed objects, whereas
option-base just takes the decoded values.
(opthelp): Parse through the (cumul ...) type syntax, so
cumulative options are printed in the help text the same way
as if they were non-cumulative.

* txr.1: Documented.

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