It's useful to be able to track both facts with a single variable, but
it also makes it more difficult to change how the buffer size scales.

As an example, Apple's implementation seems to scale the buffer size by
1.5x on growth, presumably in an attempt to reduce trips into realloc().
This might be questionable in the face of stdio buffering, but avoiding
serious churn in the line- or un-buffered case is a net positive if
doing so isn't incredibly invasive.

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