at the point pclose might receive and act on cancellation, it has
already invalidated the FILE passed to it. thus, per musl's QOI
guarantees about cancellation and resource allocation/deallocation,
it's not a candidate for cancellation.

if it were required to be a cancellation point by posix, we would have
to switch the order of deallocation, but somehow still close the pipe
in order to trigger the child process to exit. i looked into doing
this, but the logic gets ugly, and i'm not sure the semantics are
conformant, so i'd rather just leave it alone unless there's a need to
change it.