Bring back the functionality whereby an asymmetric key can be matched with a
partial match on one of its IDs.

Whilst we're at it, allow for the possibility of having an increased number of
IDs.
Reported-by: NDmitry Kasatkin <d.kasatkin@samsung.com>
Signed-off-by: NDmitry Kasatkin <d.kasatkin@samsung.com>
Signed-off-by: NDavid Howells <dhowells@redhat.com>