Enable the automatic creation of a certificate (if one does not exists)
and enable the usage by sendmail if sendmail is enabled.  Include and
document knobs to disable this feature and also set the Common Name of
the certificate created.

As the certificate is signed w/ a discarded key, it only helps prevent
Eve, but not Malory from knowing the contents of the emails.

This means that new installs (and people that use the updated freebsd.mc
file) will automaticly have STARTTLS enabled allowing incoming email to
be encrypted in most cases.

Reviewed by:	gshapiro
MFC after:	3 days
Security:	Yes, please.