Add config options for low memory servers

a 2gb ram vps is not enough to run default DTC install. allow modular server configs such as amavis/clamav/spamassassian removal or resource reduction, or replacement. clamav is using 536M ram sitting idle. amavis is using 146M x 6 processes. Also some admins may not want spf or dkim. SPF is broken on too many sites to reject mails by default, if anything it could be used to just increase spam score some.