mptcp: add REUSEADDR/REUSEPORT support

This will e.g. make 'sshd restart' work when MPTCP is used, as we will
now set this option on the listener socket instead of only the mptcp
socket (where it has no effect).

We still need to copy the setting to the master socket so that a
subsequent getsockopt() returns the expected value.

Reported-by: Christoph Paasch <cpaasch@apple.com>
Suggested-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>