why does useradd.bbclass loop retrying its commands?

["Peter A. Bigot" <pab@pabigot.com>]

The useradd, groupadd, and groupmems commands in useradd.bbclass are 
executed in a loop with up to 10 failed attempts before they give up.  
This appears to have always been the case, as long as that file has been 
present.

Is there any reason why an initial failed attempt to execute one of 
these commands would be expected to succeed on retry, other than because 
delaying failure gives a chance for concurrently executing task to 
complete and so satisfy a dependency?

(This isn't the root cause of the pseudo autobuilder failure under 
multilib, but it did look plausible for a while.)

Peter