Avoid call to slurm_get_slurmd_user_id() in _step_connect() if not slurmd.

For a stray socket, this call would cause nss_slurm to deadlock,
as any calling path that leads to slurm_conf_lock(), which will call
getpwuid(), which will re-enter the nss_slurm code, which will end up
back here but with the slurm_conf_lock already held, at which point
the process will never continue.

For nss_slurm, this means a node rebooting with stale sockets will hang
in the middle of the init process, which is a rather unpleasant experience.

So - only handle the stray socket cleanup within the slurmd process itself.

Bug 7030