mptcp: setsockopt: SO_DEBUG and no-op options

Handle SO_DEBUG and set it on all subflows.
Ignore those values not implemented on TCP sockets.

Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>