redox: make CMSG_ALIGN, CMSG_LEN, and CMSG_SPACE const functions

(backport <#4441>)
(cherry picked from commit b20a725)

Author: jackpot51

src/unix/redox/mod.rs

@@ -1014,8 +1014,19 @@ pub const PRIO_PROCESS: c_int = 0;
 pub const PRIO_PGRP: c_int = 1;
 pub const PRIO_USER: c_int = 2;
 
-// wait.h
 f! {
+    //sys/socket.h
+    pub {const} fn CMSG_ALIGN(len: size_t) -> size_t {
+        (len + mem::size_of::<size_t>() - 1) & !(mem::size_of::<size_t>() - 1)
+    }
+    pub {const} fn CMSG_LEN(length: c_uint) -> c_uint {
+        (CMSG_ALIGN(mem::size_of::<cmsghdr>()) + length as usize) as c_uint
+    }
+    pub {const} fn CMSG_SPACE(len: c_uint) -> c_uint {
+        (CMSG_ALIGN(len as size_t) + CMSG_ALIGN(mem::size_of::<cmsghdr>())) as c_uint
+    }
+
+    // wait.h
     pub fn FD_CLR(fd: c_int, set: *mut fd_set) -> () {
         let fd = fd as usize;
         let size = mem::size_of_val(&(*set).fds_bits[0]) * 8;
@@ -1228,12 +1239,9 @@ extern "C" {
     pub fn setrlimit(resource: c_int, rlim: *const crate::rlimit) -> c_int;
 
     // sys/socket.h
-    pub fn CMSG_ALIGN(len: size_t) -> size_t;
     pub fn CMSG_DATA(cmsg: *const cmsghdr) -> *mut c_uchar;
     pub fn CMSG_FIRSTHDR(mhdr: *const msghdr) -> *mut cmsghdr;
-    pub fn CMSG_LEN(len: c_uint) -> c_uint;
     pub fn CMSG_NXTHDR(mhdr: *const msghdr, cmsg: *const cmsghdr) -> *mut cmsghdr;
-    pub fn CMSG_SPACE(len: c_uint) -> c_uint;
     pub fn bind(
         socket: c_int,
         address: *const crate::sockaddr,