Provide a way to free the continuation stacks.

* share/txr/stdlib/yield.tl (sys:obtain-impl): Pass
sys:cont-free symbol to each abandoned continuation
to release its stack buffer.
(obtain): Handle the sys:cont-free symbol in the
lambda, so the initial lambda can be treated
uniformly with continuation functions.

* txr.1: Documented sys:obtain-impl.

* unwind.c (sys_cont_free_s): New symbol variable.
(cont_mark): Check for null stack pointer and avoid marking.
(revive_cont): If arg is sys:cont-free, then free the
continuation and return nil. If the continuation has a null
stack buffer, throw an error.
(uw_late_init): Initialize sys_cont_free_s.

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