Auto merge of #725 - Susurrus:mman_constants, r=alexcrichton

Add MAP_ANONYMOUS to DragonFlyBSD and FreeBSD

Author: bors

src/unix/bsd/freebsdlike/mod.rs

@@ -336,6 +336,7 @@ pub const MAP_SHARED: ::c_int = 0x0001;
 pub const MAP_PRIVATE: ::c_int = 0x0002;
 pub const MAP_FIXED: ::c_int = 0x0010;
 pub const MAP_ANON: ::c_int = 0x1000;
+pub const MAP_ANONYMOUS: ::c_int = MAP_ANON;
 
 pub const MAP_FAILED: *mut ::c_void = !0 as *mut ::c_void;