acme-acmesh: Provide a 'combined' certificate bundle as well

acme-acmesh: Provide a 'combined' certificate bundle as well

The haproxy hotplug script creates a 'combined' certificate bundle that
contains both the certificate chain and the private key. However, having a
daemon hotplug script write into CERT_DIR is not great; so let's provide
the bundle as part of the main acme framework, keeping it in $domain_dir
and just linking it into CERT_DIR. That way we can keep CERT_DIR as just a
collection of links for everything, that no consumers should need to write
into.

Also make sure to set the umask correctly so the combined file is not
world-readable (since it contains the private key).

Signed-off-by: Toke Høiland-Jørgensen <toke@toke.dk>