Impove struct cmsghdr decoding

Impove struct cmsghdr decoding

Print all cmsghdr structures in msg_control array,
not just the first one.
Change output format to be consistent with other parts of strace
where structures and arrays are printed.
Implement decoder for SCM_SECURITY message type.

* net.c (print_scm_rights, print_scm_creds, print_scm_security):
New functions.
(printcmsghdr): Use them.  Iterate over all members of the array.
* xlat/scmvals.in: Add SCM_SECURITY.
* tests/scm_rights.c (main): Pass one more descriptor to the receiver.
Set SO_PASSCRED on the receiver part.  Reserve enough space to receive
SCM_CREDENTIALS and SCM_RIGHTS.
* tests/scm_rights-fd.test: Update.