sbuild group should be created with --system switch to addgroup

[Karl Hegbloom]

Package: sbuild
Version: 0.35
Severity: normal

On my machine, the sbuild group has GID 1001, wich is in the login user group ID range.
The maintainer script for sbuild should use the --system switch to ensure that the group
it creates is in the lower range specified by Debian Policy for dynamically created GID.

-- System Information:
Debian Release: testing/unstable
  APT prefers breezy-security
  APT policy: (625, 'breezy-security'), (610, 'breezy-updates'), (600, 'breezy'), (500, 'unstable'), (500, 'testing')
Architecture: amd64 (x86_64)
Shell:  /bin/sh linked to /bin/bash
Kernel: Linux 2.6.12-9-amd64-k8-oaktree2
Locale: LANG=C, LC_CTYPE=C (charmap=ANSI_X3.4-1968) (ignored: LC_ALL set to C)

Versions of packages sbuild depends on:
ii  adduser               3.64ubuntu1        Add and remove users and groups
ii  apt                   0.6.40.1ubuntu9    Advanced front-end for dpkg
ii  dpkg-dev              1.13.10ubuntu4     Package building tools for Debian
ii  exim4                 4.52-1             metapackage to ease exim MTA (v4) 
ii  exim4-daemon-light [m 4.52-1             lightweight exim MTA (v4) daemon
ii  grep-dctrl            2.6.6ubuntu1       Grep Debian package information
ii  perl                  5.8.7-5ubuntu1     Larry Wall's Practical Extraction 
ii  sudo                  1.6.8p9-2ubuntu2.1 Provide limited super user privile

Versions of packages sbuild recommends:
ii  debootstrap               0.3.1.6ubuntu1 Bootstrap a basic Debian system
ii  fakeroot                  1.5.1ubuntu2   Gives a fake root environment

-- no debconf information