txr: bug in handling @{nil ...} variable match.

In March 2012, b7f1f4c5bbea86e288b6a4d68595c1d2d07217bd
introduced the feature that the @nil variable matches and
discards. This was incompletely implemented. Some cases
of a nil variable with modifiers fail to match.

* match.c (dest_bind): This function must correctly handle
the case when pattern is nil: it should just return bindings
without extending them. If the pattern is any nonbindable
symbol, it should indicate a failed match using t.
The logic has not been touched since 2009, at which time
an additional bogosity was introduced of calling
funcall(testfun, pattern, value) when pattern is a
non-bindable symbol. If value is a string, that could
never work. Possibly the idea is that the value could come
from a symbol-valued expression, such as one producing
a keyword symbol. We are not going to support that, unless
someone complains.

* tests/000/nilvar.txr, tests/000/nilvar.expected: New files,
providing a test case that fails without this commit.

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