socket_nif: improve support for sendmsg ctrl types

fix handling some UNIX socket cmsg_type values from erlang code:
  * timestamp <-> SCM_TIMESTAMP
  * rights <-> SCM_RIGHTS
  * credentials <-> SCM_CREDENTIALS or SCM_CREDS

'rigths' type allows passing file descriptors to another process,
which allows passing TCP sockets too, E.g.:

```
% Sender code, FD is an integer file descriptor
Ctrl = [#{level => socket, type => rights, data => <<FD:32/native>>}],
ok = socket:sendmsg(USock, #{iov => [<<"hello">>], ctrl => Ctrl}),
```

```
% Receiver side (other process)
{ok, Msg} = socket:recvmsg(U),
{ctrl := [#{level := socket, type := rights, data := Data}]} = Msg,
<<FD:32/native, Rest/binary>> = Data, % On MacOS Rest contains garbage
```

1 files changed, 10 insertions, 7 deletions

diff --git a/erts/preloaded/src/socket.erl b/erts/preloaded/src/socket.erl
index 07e720c44d..fe066e5355 100644
--- a/erts/preloaded/src/socket.erl
+++ b/erts/preloaded/src/socket.erl
@@ -615,13 +615,16 @@
         #{level := ipv6,      type := integer(),   data := binary()}       |
         #{level := integer(), type := integer(),   data := binary()}.
 -type cmsghdr_send() :: 
-        #{level := socket,    type := integer(), data := binary()} |
-        #{level := ip,        type := tos,       data := ip_tos()  | binary()} |
-        #{level := ip,        type := ttl,       data := integer() | binary()} |
-        #{level := ip,        type := integer(), data := binary()} |
-        #{level := ipv6,      type := integer(), data := binary()} |
-        #{level := udp,       type := integer(), data := binary()} |
-        #{level := integer(), type := integer(), data := binary()}.
+        #{level := socket,    type := timestamp,   data := binary()} |
+        #{level := socket,    type := rights,      data := binary()} |
+        #{level := socket,    type := credentials, data := binary()} |
+        #{level := socket,    type := integer(),   data := binary()} |
+        #{level := ip,        type := tos,         data := ip_tos()  | binary()} |
+        #{level := ip,        type := ttl,         data := integer() | binary()} |
+        #{level := ip,        type := integer(),   data := binary()} |
+        #{level := ipv6,      type := integer(),   data := binary()} |
+        #{level := udp,       type := integer(),   data := binary()} |
+        #{level := integer(), type := integer(),   data := binary()}.
 
 
 -opaque select_tag() :: atom().