optimized to avoid allocation and return lines directly out of the
stream buffer whenever possible.

the strspn call was made for every format specifier and end-of-string,
even though the expected return value was 1-2 for normal usage.
replace with simple loop.

amusingly, this cuts more than 10% off the run time of printf("a"); on
the machine i tested it on.

sadly the same optimization is not possible for snprintf without
duplicating all the pseudo-FILE setup code, which is not worth it.

this is not a standard but it's the traditional behavior and it's more
useful because the caller can reliably detect errors.

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.

close was the only cancellation point called from popen, but it left
popen with major resource leaks if any call to close got cancelled.
the easiest, cheapest fix is just to use a non-cancellable close
function.