hurd: fix definition of utsname struct

- drop the "domainname" field, as it is not actually used
- add a private "_UTSNAME_LENGTH" constant matching the helper libc one,
  to ease declaring the struct
- bump the size of the other fields to "_UTSNAME_LENGTH"

(backport <#3992>)
(cherry picked from commit 158cd30)

src/unix/hurd/mod.rs

@@ -873,12 +873,11 @@ s! {
     }
 
     pub struct utsname {
-        pub sysname: [::c_char; 65],
-        pub nodename: [::c_char; 65],
-        pub release: [::c_char; 65],
-        pub version: [::c_char; 65],
-        pub machine: [::c_char; 65],
-        pub domainname: [::c_char; 65]
+        pub sysname: [::c_char; _UTSNAME_LENGTH],
+        pub nodename: [::c_char; _UTSNAME_LENGTH],
+        pub release: [::c_char; _UTSNAME_LENGTH],
+        pub version: [::c_char; _UTSNAME_LENGTH],
+        pub machine: [::c_char; _UTSNAME_LENGTH],
     }
 
     pub struct rlimit64 {
@@ -3437,6 +3436,9 @@ pub const PTHREAD_RWLOCK_INITIALIZER: pthread_rwlock_t = pthread_rwlock_t {
 };
 pub const PTHREAD_STACK_MIN: ::size_t = 0;
 
+// Non-public helper constants
+const _UTSNAME_LENGTH: usize = 1024;
+
 const_fn! {
     {const} fn CMSG_ALIGN(len: usize) -> usize {
         len + ::mem::size_of::<usize>() - 1 & !(::mem::size_of::<usize>() - 1)