Allow (force ...) to be an assignable place.

This allows mlet variables to be assignable.

* eval.c (force_s): New global variable.
(op_modplace): Handle force form.
(me_mlet): Use force_s symbol.
(force_l): New static function.
(eval_init): Initialize force_s variable. Use it in registration
of force function.

* txr.1: Remove text in mlet definition that variables
are not assignable. Replace with note about an unspecified behavior.

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