This moves hydra->kernel_interface to charon->kernel.
The additional data can be helpful to identify the exact policy to delete.
If the (un)registering of a kernel interface (net or ipsec) fails, the plugin loader will never know, since the appropriate functions always returns TRUE. By making the (un)register functions return a boolean value, the loader can detect a failure during initializing the kernel interface and abort charon startup if desired.
The current "inbound" flag is used for two purposes: To define the actual direction of the SA, but also to determine the operation used for SA installation. If an SPI has been allocated, an update operation is required instead of an add. While the inbound flag normally defines the kind of operation required, this is not necessarily true in all cases. On the HA passive node, we install inbound SAs without prior SPI allocation.
As we now use the same reqid for multiple CHILD_SAs with the same selectors, having marks based on the reqid makes not that much sense anymore. Instead we use unique marks that use a custom identifier. This identifier is reused during rekeying, keeping the marks constant for any rule relying on it (for example installed by updown). This also simplifies handling of reqid allocation, as we do not have to query the marks that is not yet assigned for an unknown reqid.
To reassign reqids where appropriate, we explicitly allocate or confirm them centrally on the kernel-interface. Currently the state is stored in the kernel-interface wrapper for all backends, but we may add appropriate methods to each backend to implement a custom reqid allocation logic, if required.
While we can handle the first selector only in BEET mode in kernel-netlink, passing the full list gives the backend more flexibility how to handle this information.
The reqid is not strictly required, as we set the reqid with the update call when installing the negotiated SA. If we don't need a reqid at this stage, we can later allocate the reqid in the kernel backend once the SA parameters have been fully negotaited. This allows us to assign the same reqid for the same selectors to avoid conflicts on backends this is necessary.