redo previous: use same variable type for the interator as the comparison

use unsigned for iterator, fixes -Wsigned-comapre issue in module build.

netbsd32_epoll_pwait2: Dedup slightly. NFCI.

netbsd32_epoll_pwait2: Early return if no event arrived;
Do not attempt to allocate 0-byte buffer by kmem_alloc(9).

COMPAT_NETBSD32: Add support for epoll(2).