msmtp should automatically fallback to unauth for system accounts

[jdreed]

For accounts such as root, msmtp should fallback to sending unauth. Regardless of whether or not people _should_ install debathena-msmtp-mta, they _have_, and this is breaking things like cron jobs. I suspect a whitelist of accounts is the right way to deal (rather than trying to be clever with nss_nonlocal or UID). And I think the whitelist is likely "root daemon bin sys lp mail news backup and nobody"