Bug#914621: adduser: please allow a leading underscore in user names again

[Sergio Gelato]

My apologies for not having notice the closely related #521883 before 
submitting this. They can't immediately be merged because of the severity 
mismatch, and the discussion in #521883 seems to have drifted somewhat. I do 
like the idea of having a separate NAME_REGEX for system users, although that's 
definitely wishlist material while I still see this bug as a regression.