pts: Prevent creating negative user ids

pts: Prevent creating negative user ids

User ids cannot be negative (those are groups). So, error out if a
negative id is specified for createuser.

FIXES 128343

Change-Id: I6e81a419c797944bfed569aac529b9694702823d
Reviewed-on: http://gerrit.openafs.org/2993
Tested-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Phillip Moore <w.phillip.moore@gmail.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>