- Switch order of setting real uid and gid. If we set uid first, then we
  don't have enough privileges to set gid.

  This looks like a long standing bug, just recently revealed by r241852.

Approved by:	cognet
- Do not bail out if stat(2) fails with ENOENT in the spool directory. This
  happens if another atrm process removes a job while we're scanning through
  the directory.
- While at it, optimize a bit the directory scanning, so that we quit looping
  as soon as all jobs specified in argv have been dealt with.

Approved by:	cognet