Initiates a key listing operation. It sets everything up, so that subsequent invocations of op_keylist_next() return the keys in the list.
If pattern is NULL, all available keys are returned. Otherwise, pattern contains an engine specific expression that is used to limit the list to all keys matching the pattern.
If secret_only is not 0, the list is restricted to secret keys only.
The context will be busy until either all keys are received (and op_keylist_next() returns GPG_ERR_EOF), or gpgme_op_keylist_end is called to finish the operation.
The function returns the error code GPG_ERR_INV_VALUE if ctx is not a valid pointer, and passes through any errors that are reported by the crypto engine support routines.