Diagnose bad consing dot syntax like (a . b . c).

* parser.y (r_exprs): Use unique object in the terminating cons to
indicate the empty spot where the dotted cdr item will go. Check for
misplaced consing dot.  (misplaced_consing_dot_check): New static
function. Checks for the terminator atom spot being taken already.
Thus, the spot may be taken only by the very last reduction, such that
the next reduction is r_exprs -> n_exprs where the terminating atom is
processed.

* parser.c (unique_s): New global variable.
(parse_init): Initialize unique_s.

* parser.h (unique_s): Declared.

* share/txr/stdlib/place.tl (sys:placelet-1): We have a misplaced
consing dot here! It was working correctly by "terminating atom
propagation" behavior, which allowed (a . b c d) to produce
(a c d . b).  If a single terminating atom occurred in the middle of a list, it
was promoted to the end.

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