Rework debug3 messages in _handle_request.

Use __func__, and list the function name first in the message.

Drop one redundant message printing the request number - all paths
through the switch statement will print this out in some form.

Remove a ternary used to print SLURM_SUCCESS/SLURM_FAILURE and
print the raw return value. If you're staring at debug3 logs,
you should hopefully know how to interpret these values. :)