Fix const errors caused by find_keys() changes.

Fix const errors caused by find_keys() changes.

Part 4 of the oppenc series changed the keyID type to const as part
of some cleanup changes.  At the time, that only propagated to
crypt_getkeybystr(), but with the fingerprint changes, this is starting
to propagate too far.

Create a separate non-const variable to deal with the crypt hook value
and revert crypt_getkeybystr()'s parameter to non-const.