selftest: af_unix: Make SCM_RIGHTS into OOB data.

scm_rights.c covers various test cases for inflight file descriptors
and garbage collector for AF_UNIX sockets.

Currently, SCM_RIGHTS messages are sent with 3-bytes string, and it's
not good for MSG_OOB cases, as SCM_RIGTS cmsg goes with the first 2-bytes,
which is non-OOB data.

Let's send SCM_RIGHTS messages with 1-byte character to pack SCM_RIGHTS
into OOB data.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: Michal Luczaj <mhal@rbox.co>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>