sdiag - handle unknown users

Instead of giving user as "nobody", give user numeric id. The RPC stats
last the lifetime of the slurmctld process and thus may have users that
have been removed.

There is no performance benefit from caching users in sdiag as it will
only lookup each user once. Caching adds qsort insert cost to every
user for no benefit. Swapped from non-cached call to direct user lookup.

Bug 7845.