RFC: [PATCH] SCM_CREDS support 2(2)

[Svante Signell]

Hi,

Patch 2(2) on SCM_CREDS support for GNU/Hurd.

This patch is optional. kFreeBSD dos not support this case (but Linux
is).

This patch implements the last cases in the test code sent to the list
in September 2013, options <-n> and <-z> see
http://lists.debian.org/debian-hurd/2013/09/msg00034.html 

Options:
no flag:
-z: send a zero credentials byte as data (used by dbus)
-n: don't create explicit credentials structure

1) Sent credentials and PID are correct:
./scm_cred_recv &
./scm_cred_send -n
Linux: OK, only sent ids
kFreeBSD: nOK; ERROR: bad cmsg header cmhp == NULL, data: OK
Hurd: same as kFreeBSD

./scm_cred_recv -z&
./scm_cred_send -nz
Linux: OK, same as above
kFreeBSD: nOK; ERROR: bad cmsg header cmhp == NULL, data: OK
Hurd: OK, sent IDs + groups

2) When no credentials structure is sent the behaviour is different:
Linux:
-n: received credentials are created
-zn: received credentials ones are created

kFreeBSD: 
-n: ERROR: bad cmsg header cmhp == NULL
-zn: ERROR: bad cmsg header cmhp == NULL

Hurd:
-n: ERROR: bad cmsg header cmhp == NULL
-zn: received credentials are created

scm_creds-sendmsg.c_2.diff
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