Fix a thundering herd problem in pipes.

Wake only one waiter when data becomes available, not all of them.
Waking them all is not a usual case, but turns up with make's job
token pipes. (Probably make's job signalling scheme should also be
revised, assuming rillig hasn't already done that, but that's a
separate issue.)

This change will not do us much good for the moment because we don't
distinguish cv_signal from cv_broadcast for interruptible sleeps, but
that's also a separate problem.

Seen on FreeBSD; from mjg at freebsd a couple months ago. Patch was
mine (iirc) but the real work in this sort of thing is discovering the
problem.

(dholland)