The updated instruction is not very user-friendly. I think the right
solution is to wrap the `/dns` commands in a `tools/dns.py` style
script, along the lines of `tools/mail.py`.

Intended to be the simplest auth possible: every time the service
starts, a random key is written to `/var/lib/mailinabox/api.key`. In
order to authenticate to the service, the client must pass the contents
of `api.key` in an HTTP basic auth header. In this way, users who do not
have read access to that file are not able to communicate with the
service.

Don't check NS records for now because they will only appear on zones.
If a hostname is a subdomain on a zone and not itself a zone, it will
lack NS records.

Also stop testing for ADSP, which we dropped in 126ea94c.

It just echos back the subject given to it.

manage the nginx conf in the management daemon too so we can have nginx operate on all domains that we serve mail for

drop support for ADSP which since last November is no longer recommended per http://datatracker.ietf.org/doc/status-change-adsp-rfc5617-to-historic/

use DANE when sending mail: if the recipient MX has a DANE TLSA record in DNS then Postfix will necessarily encrypt the mail in transport

Postfix has a tls_security_level called "dane" which uses DNS-Based Authentication of Named Entities (DANE)
to require, if specified in the DNS of the MX host, an encrpyted connection with a known certificate.

This commit adds TLSA records.

run bind9 on the loopback interface for ensuring we are using a DNSSEC-aware nameserver to resolve our own DNS queries (i.e. when sending mail) since we can't trust that the network configuration provided for us gives us a DNSSEC-aware DNS server

see #71

dont create a separate zone for PUBLIC_HOSTNAME if it is a subdomain of another zone (hmm, this is a general principle that could apply to any two domains the box is serving)

* sign zones
* in a cron job, periodically re-sign zones because they expire (not tested)

two more cases of shelling out external programs in a more secure way, see cecda9ce

Now that dns_update is a part of the management daemon, we no
longer are using STORAGE_ROOT/dns for anything.

Duplicity will manage the process of creating incremental backups for us.
Although duplicity can both encrypt & copy files to a remote host, I really
don't like PGP and so I don't want to use that.

Instead, we'll back up to a local directory unencrypted, then manually
encrypt the full & incremental backup files. Synchronizing the encrypted
backup directory to a remote host is a TODO.

Support dual-stack IPv4/IPv6 mail servers

Testing showed that it may take a few seconds for the default values to
populate.  If the help text is shown, “Enter the public IP address…,”
but no prompt is shown, the user may get confused and try to enter the
IP address before mailinabox has had a chance to figure out and display
a suitable default value.

This re-implements part of PR #69 by @mkropat, who wrote:

By default, Postfix adds a Received header — on all mail that you send —
that lists the IP of the device you sent the mail from.  This feature is
great if you're a mail provider and you need to debug why one user is
having sending issues.  This feature is not so great if you run your own
mail server and you don't want every recipient of every email you send
to know the device and IP you sent the email from.

To limit this filtering to outgoing mail only, we apply the filters just
to the submission port.  See these guides [1] [2] for more context.

  [1] http://askubuntu.com/a/78168/11259
  [2] http://www.void.gr/kargig/blog/2013/11/24/anonymize-headers-in-postfix/

Addresses #3

Added support by adding parallel code wherever `$PUBLIC_IP` was used.
Providing an IPv6 address is completely optional.

Playing around on my IPv6-enabled mail server revealed that — before
this change — mailinabox might try to use an IPv6 address as the value
for `$PUBLIC_IP`, which wouldn't work out well.

* using it to enable the Postfix submission service
* per @mkropat's suggestion in #69, set an option to distinguish submission from regular smpd in syslog by giving submission a new name (doing this here to test that editconf is working right on master.cf)

adding a new script archive_conf_files.sh to dump the contents of all files modified by editconf.py so testing is easier

Protect private key from being world-readable

Postfix, Dovecot, and nginx all read the key file while they're running
as root — before dropping permissions — so no authorization is needed on
the private key file beyond being root-readable.