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

Author: bors

src/unix/bsd/freebsdlike/freebsd/mod.rs

@@ -703,6 +703,7 @@ pub const SHM_LOCK: ::c_int = 11;
 pub const SHM_UNLOCK: ::c_int = 12;
 pub const SHM_STAT: ::c_int = 13;
 pub const SHM_INFO: ::c_int = 14;
+pub const SHM_ANON: *mut ::c_char = 1 as *mut ::c_char;
 
 // The *_MAXID constants never should've been used outside of the
 // FreeBSD base system.  And with the exception of CTL_P1003_1B_MAXID,