libstdio: sync bits of vfprintf from APE

* Add the %ll length modifier,
* Convert nil to "<nil>" under %s (not in APE),
* Cast void* to uintptr under %p,
* Use "0x" hex prefix under %p,
* Fix manual page mentions of %P to %p,
* Fix empty result for fp conversions,
* Fix zero padding of left-aligned fp conversions,
* Remove deprecated #pragma ref uses.

Most of these were introduced in APE prior to 9front.

I've omitted the %z conversion specifier since Plan 9 code
rarely uses the usize type. This may need to be added later
for the benefit of native ports of alien code.

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