sysusers.generate-pre.sh: properly escape quotes in description strings
    
    ... (rhbz#2104141)
    
    In the first version, I wanted to use POSIX quotes with $''. But that required
    'printf %q', which brings in a dependency on coreutils.
    
    Following mcr0mmand's suggestion, ${foo@Q} is used instead, which should work
    equivalently, and does not require anything new.
    
    Tested with 'sysusers.generate-pre.sh /usr/lib/sysusers.d/*conf'. The output is
    the same before and after, apart from the dovecot user with a quote.