refactor the mail setup scripts

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.

setup/start.sh

@@ -182,7 +182,9 @@ EOF
 . setup/system.sh
 . setup/ssl.sh
 . setup/dns.sh
-. setup/mail.sh
+. setup/mail-postfix.sh
+. setup/mail-dovecot.sh
+. setup/mail-users.sh
 . setup/dkim.sh
 . setup/spamassassin.sh
 . setup/web.sh