allow specifying cmd-cnt-name and cmd-max-name in netlink specs, in
accordance with Documentation/userspace-api/netlink/c-code-gen.rst.

Use cmd-cnt-name and attr-cnt-name in the mptcp yaml spec and in the
corresponding uAPI headers, to preserve the #defines we had in the past
and avoid adding new ones.

v2:
 - squash modification in mptcp.yaml and MPTCP uAPI headers
Suggested-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Link: https://lore.kernel.org/r/12d4ed0116d8883cf4b533b856f3125a34e56749.1698415310.git.dcaratti@redhat.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>