eval: bugs/regressions in handling nil in param expansion.

Due to a number of changes since December 2016, we have two
problems in the evaluator: (lambda (nil)) is accepted rather
than complaining that nil is not bindable; and
(tree-bind (()) '(3)) silently proceeds rather than
diagnosing that the atom 3 doesn't match the empty pattern ().

* eval.c (expand_opt_params_rec, expand_params_rec): When not
expandin macro-style params, check for a parameter not being
bindable, and also avoid recursion entirely when not doing
macro style.
(bind_macro_params): Don't assume that an atom parameter is
a variable, but rather tha a non-list parameter is
a variable. Otherwise we bind nil rather than treating it
as an empty sub-pattern. Before the December 2016 change
(744340ab), we were checking bindablep here; the idea was to
hoist the detailed checking to expansion time. But then the
pattern versus variable distinction was bungled here.

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