We have to be a little careful, as we can't use Gnulib in the public
<pipeline.h> header; but we can rely on it for internal compilation
units.

* lib/pipeline.c (passthrough): Use _GL_UNUSED rather than
PIPELINE_ATTR_UNUSED.
* tests/basic.c (pre_exec): Likewise.
* tests/common.h (MAIN): Likewise.
* tests/inspect.c (pid_helper): Likewise.
* tests/pump.c (tee_source): Likewise.
* lib/pipeline.h (PIPELINE_ATTR_UNUSED): Note that this is unused, but
preserved to avoid breaking (even undocumented) API.