Fix writeNSS passwd, shadow, and group lines
    
    Between 6.2.10 and 7.0.0, changes to `writeNSS` prevented the
    `users` variable being set to anything other than "", so the
    `passwd`, `shadow` and `group` lines written by authconfig would
    always have no values.
    
    This fixes that, and rejigs the code a little to be clearer and
    avoid needless initialization of variables to the empty string
    and leading spaces.