agent: Replace es_mopen by es_fopenmem for ssh.

* agent/command-ssh.c (ssh_read_key_public_from_blob): Use
es_fopenmem.
(ssh_handler_request_identities): Ditto.
(ssh_request_process): Ditto.

es_fopenmem is easier to understand than the more general function
es_mopen. Thus we better use the former for clarity.