nshlib: enable O_CLOEXEC explicit to avoid potential fd leak

leaking here means fork/vfork will duplicate fd without O_CLOEXEC flag
to the child process.

Signed-off-by: wanggang26 <wanggang26@xiaomi.com>

nshlib/nsh_syscmds.c

@@ -598,7 +598,7 @@ static int cmd_rpmsg_once(FAR struct nsh_vtbl_s *vtbl,
       return ERROR;
     }
 
-  fd = open(path, 0);
+  fd = open(path, O_CLOEXEC);
   if (fd < 0)
     {
       nsh_output(vtbl, g_fmtarginvalid, path);