Algorithm rollover will require four keys so introduce KEY4.
Also it requires to look at key files for multiple algorithms so
change getting key ids to be algorithm rollover agnostic (adjusting
count checks). The algorithm will be verified in check_key so
relaxing 'get_keyids' is fine.
Replace '${_alg_num}' with '$(key_get KEY[1-4] ALG_NUM)' in checks
to deal with multiple algorithms.
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