This directly fixes the problem that monkeysphere-host was failing to
import the style of SSH host key that was generated by default by
ssh-keygen.

As a side effect, we can now support all the host key algorithms that
are supported by both gpg-agent (in its ssh-agent implementation) and
ssh-add.

The lockfile and the dancing around the gpg-agent run in
$GNUPGHOME_HOST is a bit awkward.  It would be nicer to use an
ephemeral GnuPG homedir as recommended in the "Unattended Usage"
section of the gnupg info pages, but to do that we'd need to be able
to extract the secret key, which is blocked by
https://dev.gnupg.org/T4490