unix/modsocket: Add IP ADD and DROP MEMBERSHIP to socket constants.

Add the IP_ADD_MEMBERSHIP and IP_DROP_MEMBERSHIP to modsocket in the Unix
port so that the directives take on the values defined in the system
headers. This is needed because the values of these directives are
different for MacOS vs other Unix systems.

Fixes issue #8456.

Signed-off-by: Alex Tran <alex.t.tran@gmail.com>

ports/unix/modsocket.c

@@ -709,6 +709,9 @@ static const mp_rom_map_elem_t mp_module_socket_globals_table[] = {
     C(SO_KEEPALIVE),
     C(SO_LINGER),
     C(SO_REUSEADDR),
+
+    C(IP_ADD_MEMBERSHIP),
+    C(IP_DROP_MEMBERSHIP),
 #undef C
 };