dma - Fix security hole (#46)

dma - Fix security hole (#46)

Affecting DragonFly 4.6 and earlier, Matt Dillon fixed this in base after
finding out from BSDNow Episode 152. Comments following were from his commit
which explains better than I. Just taking his change and putting it here as well.

* dma makes an age-old mistake of not properly checking whether a file
  owned by a user is a symlink or not, a bug which the original mail.local
  also had.

* Add O_NOFOLLOW to disallow symlinks.

Thanks-to: BSDNow Episode 152, made me dive dma to check when they talked
           about the mail.local bug.