sfio: use a more portable way of determining page size

sfio: use a more portable way of determining page size

This code was using a legacy API, `getpagesize`, to opportunistically find the
system virtual memory page size. This change updates to the newer POSIX
`sysconf` API that is available almost everywhere, and adds a workaround for
Windows. This code now has an accurate understanding of the page size on
platforms that did not have `getpagesize` (of which, Windows was one), rather
than falling back on 8192.