Port t_mmap.c to FreeBSD

- Add needed headers for the testcases
- Omit mmap_block on non-NetBSD OSes
- Use "security.bsd.map_at_zero" instead of "vm.user_va0_disable"

Submitted by: pho
Sponsored by: EMC / Isilon Storage Division
Omit the pollts testcases on FreeBSD

Sponsored by: EMC / Isilon Storage Division
Omit all of the testcases as revoke(2) is only implemented on devfs(5)

Submitted by: pho
Sponsored by: EMC / Isilon Storage Division
Mark signo __unused in handler(..)

Sponsored by: EMC / Isilon Storage Division
- Omit the poll testcases on FreeBSD (they require pollts)
- Add necessary headers for the testcases

Sponsored by: EMC / Isilon Storage Division