Changed the default age weight to zero in the mf plugin

  affinity bits don't get cleared by default for direct use of the launch API
  (as used by poe). 
If a zero value is registerd for task launch, reset to 1 and avoid an empty bitmap
  and subsequent task binding error.

   otherwise a batch job is unable to create an sgi_jobs container for the
   proctrack/sgi_jobs plugin

Revise task exit log messages from slurmstepd to be clearer,
and include the termination signal if WIFSIGNALED, and a
message "(core dumped)" if a corefile was generated. The
extra timestamp is also removed.

  event that the hardware does not match the configuration information.

This patch abstracts the establishment of slurm user credentials
in slurmctld when SlurmUser != root. It adds a call to setgid()
to adopt the SLURM user's primary group, instead of running with
whatever primary group was in effect when invoking the slurmctld
executable. Also, errors in establishing user credentials are
fatal errors, instead of just warnings.

This is a minor security enhancement.