mailer.conf: Fix some issues.

mailer.conf: Fix some issues.

* dma(8) accepts being called as newaliases, causing it to parse the
  aliases file, thereby checking its integrity, and erroring if
  something is wrong, so allow it to do that. It also has code to
  deal with being called as hoststat or purgestat, so allow that too.

* Use true(1)'s full path since /usr/bin isn't necessarily in the
  path in all situations (such as when called from rc scripts).