selftest: af_unix: Add SO_OOBINLINE test cases in msg_oob.c

When SO_OOBINLINE is enabled on a socket, MSG_OOB can be recv()ed
without MSG_OOB flag, and ioctl(SIOCATMARK) will behaves differently.

This patch adds some test cases for SO_OOBINLINE.

Note the new test cases found two bugs in TCP.

  1) After reading OOB data with non-inline mode, we can re-read
     the data by setting SO_OOBINLINE.

  #  RUN           msg_oob.no_peek.inline_oob_ahead_break ...
  # msg_oob.c:146:inline_oob_ahead_break:AF_UNIX :world
  # msg_oob.c:147:inline_oob_ahead_break:TCP     :oworld
  #            OK  msg_oob.no_peek.inline_oob_ahead_break
  ok 14 msg_oob.no_peek.inline_oob_ahead_break

  2) The head OOB data is dropped if SO_OOBINLINE is disabled
     if a new OOB data is queued.

  #  RUN           msg_oob.no_peek.inline_ex_oob_drop ...
  # msg_oob.c:171:inline_ex_oob_drop:AF_UNIX :x
  # msg_oob.c:172:inline_ex_oob_drop:TCP     :y
  # msg_oob.c:146:inline_ex_oob_drop:AF_UNIX :y
  # msg_oob.c:147:inline_ex_oob_drop:TCP     :Resource temporarily unavailable
  #            OK  msg_oob.no_peek.inline_ex_oob_drop
  ok 17 msg_oob.no_peek.inline_ex_oob_drop

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>