[hurd,commited] hurd sendmsg: Fix warning on calling CMSG_*HDR

[Florian Weimer]

* Andreas Schwab:

> On Dez 29 2019, Samuel Thibault wrote:
>
>> diff --git a/sysdeps/mach/hurd/sendmsg.c b/sysdeps/mach/hurd/sendmsg.c
>> index 0c19b3223c..3d7317cec4 100644
>> --- a/sysdeps/mach/hurd/sendmsg.c
>> +++ b/sysdeps/mach/hurd/sendmsg.c
>> @@ -108,7 +108,7 @@ __libc_sendmsg (int fd, const struct msghdr
>> *message, int flags)
>>  
>>    /* Allocate enough room for ports.  */
>>    cmsg = CMSG_FIRSTHDR (message);
>> -  for (; cmsg; cmsg = CMSG_NXTHDR (message, cmsg))
>> +  for (; cmsg; cmsg = CMSG_NXTHDR ((struct msghdr *) message, cmsg))
>
> Why is the first argument of __cmsg_nxthdr not const?

Some programmers expect that CMSG_NXTHDR can be used to construct
ancillary data.  See bug 13500.