freebsd: add sockcred/SOCKCREDSIZE

Like on NetBSD, but without a pid field in the struct.

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

@@ -107,6 +107,15 @@ s! {
         pub msg_hdr: ::msghdr,
         pub msg_len: ::ssize_t,
     }
+
+    pub struct sockcred {
+        pub sc_uid: ::uid_t,
+        pub sc_euid: ::uid_t,
+        pub sc_gid: ::gid_t,
+        pub sc_egid: ::gid_t,
+        pub sc_ngroups: ::c_int,
+        pub sc_groups: [::gid_t; 1],
+    }
 }
 
 s_no_extra_traits! {
@@ -1136,6 +1145,15 @@ f! {
             as ::c_uint
     }
 
+    pub fn SOCKCREDSIZE(ngrps: usize) -> usize {
+        let ngrps = if ngrps > 0 {
+            ngrps - 1
+        } else {
+            0
+        };
+        ::mem::size_of::<sockcred>() + ::mem::size_of::<::gid_t>() * ngrps
+    }
+
     pub fn uname(buf: *mut ::utsname) -> ::c_int {
         __xuname(256, buf as *mut ::c_void)
     }