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


git-svn-id: svn+ssh://svn.freebsd.org/base/head@273525 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f