Description:

Added the `backupcyrusd` protocol daemon from Fastmail.


Documentation:

The command is documented in an RST manpage, and the protocol itself
is documented in comments in the backupcyrusd.c file itself.


Config changes:

No changes, it's a standard daemon which can be invoked like any other
daemon in cyrus.conf.


Upgrade instructions:

Nothing to change, but only enable this feature if you're sure your host
and network is secure, since it runs unauthenticated and allows read access
to the data of all users on the server.


GitHub issue:

If theres a github issue number for this, put it here.
