o Remove all instances of constant poll() timeouts (only timeout if needed)

 o Move checks for launch or exit timeouts from IO thread into msg.c
 o Added new "undocumented" options --max-launch-time and --max-exit-timeout
   for use in testing.
 o Do not wantonly terminate IO thread when job moves into state
   TERMINATED, only exit on FORCETERM. (this allows collection of error
   messages from slurmds)
 o Small amount of code cleanup in io.c, signals.c