Attempt to clean up the slurm_msg_t initializer function.

slurm_init_slurm_msg is replaced by slurm_msg_t_init and slurm_msg_t_copy.

slurm_msg_t_init COMPLETELY initializes the structure, so it needs to be
called BEFORE setting any variables within the structure.

It is not clear if slurm_msg_t_copy is even necessary, it is only used
in three places.