Auto merge of #2125 - asomers:MAP_GUARD, r=JohnTitor

Add MAP_GUARD on FreeBSD.

It was added in FreeBSD 11.1.

Author: bors

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

@@ -447,6 +447,8 @@ pub const NI_NUMERICSCOPE: ::c_int = 0x00000020;
 pub const Q_GETQUOTA: ::c_int = 0x700;
 pub const Q_SETQUOTA: ::c_int = 0x800;
 
+pub const MAP_GUARD: ::c_int = 0x00002000;
+
 pub const POSIX_FADV_NORMAL: ::c_int = 0;
 pub const POSIX_FADV_RANDOM: ::c_int = 1;
 pub const POSIX_FADV_SEQUENTIAL: ::c_int = 2;