std.c.openbsd: remove "msg_" prefixes from structs

Reapplies 8f14431bc8 which was reverted in
95e2605d30.

lib/std/c/openbsd.zig

@@ -251,48 +251,36 @@ pub const EAI_MAX = 15;
 
 pub const msghdr = extern struct {
     /// optional address
-    msg_name: ?*sockaddr,
-
+    name: ?*sockaddr,
     /// size of address
-    msg_namelen: socklen_t,
-
+    namelen: socklen_t,
     /// scatter/gather array
-    msg_iov: [*]iovec,
-
-    /// # elements in msg_iov
-    msg_iovlen: c_uint,
-
+    iov: [*]iovec,
+    /// # elements in iov
+    iovlen: c_uint,
     /// ancillary data
-    msg_control: ?*anyopaque,
-
+    control: ?*anyopaque,
     /// ancillary data buffer len
-    msg_controllen: socklen_t,
-
+    controllen: socklen_t,
     /// flags on received message
-    msg_flags: c_int,
+    flags: c_int,
 };
 
 pub const msghdr_const = extern struct {
     /// optional address
-    msg_name: ?*const sockaddr,
-
+    name: ?*const sockaddr,
     /// size of address
-    msg_namelen: socklen_t,
-
+    namelen: socklen_t,
     /// scatter/gather array
-    msg_iov: [*]const iovec_const,
-
-    /// # elements in msg_iov
-    msg_iovlen: c_uint,
-
+    iov: [*]const iovec_const,
+    /// # elements in iov
+    iovlen: c_uint,
     /// ancillary data
-    msg_control: ?*const anyopaque,
-
+    control: ?*const anyopaque,
     /// ancillary data buffer len
-    msg_controllen: socklen_t,
-
+    controllen: socklen_t,
     /// flags on received message
-    msg_flags: c_int,
+    flags: c_int,
 };
 
 pub const Stat = extern struct {