Add hooks to parse and format the MP_CAPABLE option.

This option is handled according to MPTCP version 0 (RFC6824).
MPTCP version 1 MP_CAPABLE (RFC6824bis/RFC8684) will be added later in
coordination with related code changes.
Co-developed-by: NMatthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: NMatthieu Baerts <matthieu.baerts@tessares.net>
Co-developed-by: NFlorian Westphal <fw@strlen.de>
Signed-off-by: NFlorian Westphal <fw@strlen.de>
Co-developed-by: NDavide Caratti <dcaratti@redhat.com>
Signed-off-by: NDavide Caratti <dcaratti@redhat.com>
Signed-off-by: NPeter Krystad <peter.krystad@linux.intel.com>
Signed-off-by: NChristoph Paasch <cpaasch@apple.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>