Feature: implement pass-through mode.

In pass-through mode, pw can be used in the middle of a pipeline,
or redirected to a file or device. Then instead of discarding
data, it copies from standard input to standard output.

pw's implementation assumes that standard output is the terminal,
and uses some functions that operate implicitly on standard output.
To avoid changing all that code, dup2 calls are used to rearrange
the file descriptors. The tty descriptor opened from /dev/tty is
installed as standard output, and the original standard output
(the device, file or pipe to which data is to be passed through)
is tied to a dedicated stream held in the local variable out.
Whenever out is not null, bytes are to be sent to it rather than
discarded.


In pass-through mode, though, standard output is not the
terminal.

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