streams: close-stream only caches non-nil result.

This is motivated by trying to implement a struct delegate
stream which performs reference counting in close, in
order to close the real stream when the count hits zero.

The caching behavior of close-stream is a problem.

* stream.c (strm_base_init): Initialize close_result to nil,
rather than nao.
(strm_base_mark): Don't check close_result for nao.
(close_stream): Suppress the call to op->close if close_result
has a non-nil value, rather than a value other than nao.

* tests/018/close-delegate.tl,
* tests/018/close-delegate.expected: New files.

* txr.1: Document that only a non-nil return is cached
by close-stream.

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