As the scripts keep growing, it's time to split them up to
keep them understandable.

This splits mail.sh into mail-postfix.sh, mail-dovecot.sh,
and mail-users.sh, which has all of the user database-related
configurations shared by Dovecot and Postfix. Also from
spamassassin.sh the core sieve configuration is moved into
mail-dovecot.sh and the virtual transport setting is moved
into mail-postfix.sh.

Also revising one of the sed scripts in mail-dovecot to
not insert a new additional # at the start of a line each
time the script is run.

closes #94

in cf7053c1 I allowed editconf.py to insert a setting where we find it already commented-out in order to get an nginx configuration line in the right place, but it wasn't quite right because when run again we would insert the setting a second time

the new migration management in c8856f10 left out the part where we actually keep the system's current MIGRATIONID... it was being lost when setup/start.sh was re-run

whats_next: Allow the PRIMARY_HOSTNAME to not have an MX because the default value means the domain itself, which is what we want anyway

clarify that an SSL cert can remain self-signed on the non-primary domains if the domain isn't being used for web

migrate the SSL certificates path for non-primary certs to a new layout using a new migration script

move the SSH password login check into whats_next.py (it used to be in start.sh and then moved to an unused script when it became a problem for Vagrant)

Moved the configuration to a single YAML file, rather than one per domain, to be clearer.

re-does 33f06f29

if we handle mail for both a domain and any subdomain, only create a zone for the domain and put the subdomain's DNS records in the main domain's zone file

first pass at a management tool for checking what the user must do to finish his configuration: set NS records, DS records, sign his certificates, etc.

dont ask for a CSR country code on second runs because the CSR is already generated and any new country code won't be used anyway