ssh: Allow to define the order in which keys are returned.

* agent/findkey.c (public_key_from_file): Add arg r_sshorder.
(agent_ssh_key_from_file): Ditto.
* agent/command-ssh.c (struct key_collection_item_s): New.
(struct key_collection_s): New.
(search_control_file): Add art r_lnr.
(add_to_key_array): New.
(free_key_array): New.
(compare_key_collection_items): New.
(ssh_send_available_keys): Rewrite to return the keys in the user
given order.

We now first return the keys from active cards, followed by keys
listed in sshcontrol, finally from those with the "Use-for-ssh" key
attribute. Keys from active cards are returned sorted by their S/N.
Keys from sshcontrol are returned in the order they are given in that
file. Use-for-ssh keys are ordered by the value assigned to that key
attribute. The values for the latter are clamped at 99999.

• GnuPG-bug-id: T6212