ssh-agent: check for loaded id filenames first (#7521)

This change makes the plugin check if an identity is loaded by looking
first at the key filename reported by `ssh-add -l`. This fixes the use
case where ssh-keygen is not able to output the fingerprint of a key,
such as the one reported on #7516.

Now, for an identity to be passed onto ssh-add, it has to fail the
match for a loaded identity, both filename and signature.