Auto merge of #837 - myfreeweb:freebsd-shm-anon, r=alexcrichton
Auto merge of #837 - myfreeweb:freebsd-shm-anon, r=alexcrichton

Add SHM_ANON for FreeBSD

This is a constant for `shm_open` that is used instead of the address to create an anonymous shared memory mapping.

https://github.com/freebsd/freebsd/blob/a9656e45a9d1cc0fc5343dd941106e0b823ff770/sys/sys/mman.h#L168

rust-lang-nursery/libc,rust-lang/libc,cactorium/libc,cactorium/libc,rust-lang-nursery/libc,rust-lang/libc,rust-lang-nursery/libc,cactorium/libc,rust-lang/libc