Diff reduce with upstream and consolidate #ifdefs where possible

Note that sys/types.h is required on FreeBSD for kqueue(2), unlike NetBSD

MFC after: 12 days
X-MFC with: r305358
Sponsored by: Dell EMC Isilon

- Omit the poll testcases on FreeBSD (they require pollts)
- Add necessary headers for the testcases

Sponsored by: EMC / Isilon Storage Division