ssh: Improve key lookup for many keys.

* agent/command-ssh.c: Remove dirent.h.
(control_file_s): Add struct item.
(rewind_control_file): New.
(search_control_file): Factor code out to ...
(read_control_file_item): New.
(ssh_handler_request_identities): Change to iterate over entries in
sshcontrol.

Formerly we scanned the private key directory for matches of entries
in sshcontrol. This patch changes it to scan the sshcontrol file and
thus considers only keys configured there. The rationale for this is
that it is common to have only a few ssh keys but many private keys.
Even if that assumption does not hold true, the scanning of the
sshcontrol file is faster than reading the directory and only then
scanning the ssh control for each directory entry.