KEYS: Limit ABI change in 4.13.10

KEYS: Limit ABI change in 4.13.10

Commit 363b02dab09b ("KEYS: Fix race between updating and finding a
negative key") rearranged various state members in struct key, resulting
in an ABI change for all keys APIs.

Only the keys subsystem and key type implementation use this state
information, so we can limit the ABI break to them:

- Renumber the other flags back to their old values
- Hide the deletion of the reject_error field from genksyms (it was only
  used inside the keys subsystem)
- Move the new state field to the end of the structure and hide it from
  genksyms
- Rename the register_key_type() function, so newly built key type
  modules will only load on top of the new keys subsystem while old
  key type modules will only load on top of the old keys subsystem

Gbp-Pq: Topic debian
Gbp-Pq: Name keys-limit-abi-change-in-4.13.10.patch