Use MAXALIGN size/alignment to guarantee that later uses of memory are
aligned correctly. E.g. epoll_event might need 8 byte alignment on some
platforms, but earlier allocations like WaitEventSet and WaitEvent might
not sized to guarantee that when purely using sizeof().

Found by myself while testing on an Sun Ultra 5 (Sparc IIi) with some
editorializing by Andres Freund.

In passing fix a couple typos in the area