streams: *stdnull* lazily opens /dev/null

The *stdnull* stream has been purely a stream-level
abstraction. To make it useful for redirecting real file
descriptors around the execution of external programs,
we endow it with the ability to open /dev/null when it is
asked to provide its file descriptor.

* stream.c (struct dev_null): New structure.
(dev_null_close, dev_null_get_fd, dev_null_get_prop): New
static functions.
(null_ops): Wire in the above functions instead of null_close,
null_get_fd and null_get_prop. We need new functions because
null_close and others do not belong to just the null stream;
they are base operations used by other streams as default
implementations for some kinds of unimplemented functions.
(make_null_stream): Alocate a struct dev_null instead of a
struct strm_base. Set the fd to -1.

* txr.1: Documented.

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