tzdata update keeps changing my timezone

[ Impact ]

Users can select some time zones in debconf that the tzdata debconf script will replace on the next run. See "Original report" for more explanation. Either those time zones should not be selectable in debconf or they should not be replaced.

[ Test Plan ]

Select US/Central as time zone and run dpkg-reconfigure:

```
sudo ln -sf /usr/share/zoneinfo/US/Central /etc/localtime
echo "US/Central" | sudo tee /etc/timezone
sudo dpkg-reconfigure -fnoninteractive tzdata
```

The output of dpkg-reconfigure should say that the current default time zone is still US/Central.

Besides this manual test, test_timezone_conversions is backported to check that there are no timezones which can be selected, but will be converted.

[ Where problems could occur ]

Besides added tests, the debconf part of tzdata is touched and could cause issues. The change does not touch the actual timezones.

[ Original report ]

Other than the fact that Chicago, Illinois is in US Central Timezone, I have no relationship to Chicago. I am not in Chicago. I do not want to be in Chicago. I am in the Central Timezone. I am physically closer to other cities mentioned in tzdata. Selecting America/Chicago versus US/Central is not easier for me nor is it more intuitive. I understand that the upstream tzdata package goes a long way in documenting all of the weirdness associated with timezones and different localities' treatment of timezones and daylight "savings" time. However, from an end-user perspective, the distribution should place no special importance or preference on one timezone specification or another. Likewise, if a user chooses a timezone, the system should never change it.

To reproduce the issue:

# Manually select US/Central
me@ubuntu:~$ sudo dpkg-reconfigure tzdata
[sudo] password for me:

Current default time zone: 'US/Central'
Local time is now: Wed Apr 27 16:05:02 CDT 2011.
Universal Time is now: Wed Apr 27 21:05:02 UTC 2011.

# Validation that selection was accepted
me@ubuntu:~$ cat /etc/timezone
US/Central

me@ubuntu:~$ md5sum /etc/localtime /usr/share/zoneinfo/US/Central
6540624294b1193e22ed7a15692f2de7 /etc/localtime
6540624294b1193e22ed7a15692f2de7 /usr/share/zoneinfo/US/Central

me@ubuntu:~$ debconf-show tzdata
debconf: DbDriver "passwords" warning: could not open /var/cache/debconf/passwords.dat: Permission denied
  tzdata/Zones/Australia:
* tzdata/Zones/US: Central
  tzdata/Zones/Asia:
* tzdata/Zones/Etc: UTC
  tzdata/Zones/SystemV:
  tzdata/Zones/Arctic:
  tzdata/Zones/Pacific:
  tzdata/Zones/Antarctica:
  tzdata/Zones/Europe:
  tzdata/Zones/Africa:
* tzdata/Zones/America: Chicago
* tzdata/Areas: US
  tzdata/Zones/Atlantic:
  tzdata/Zones/Indian:

# Force the package to reinstall
me@ubuntu:~$ aptitude reinstall tzdata
The following packages will be REINSTALLED:
  tzdata
0 packages upgraded, 0 newly installed, 1 reinstalled, 0 to remove and 0 not upgraded.
Need to get 0 B/658 kB of archives. After unpacking 0 B will be used.
Preconfiguring packages ...
(Reading database ... 168356 files and directories currently installed.)
Preparing to replace tzdata 2011g-0ubuntu0.11.04 (using .../tzdata_2011g-0ubuntu0.11.04_all.deb) ...
Unpacking replacement tzdata ...
Setting up tzdata (2011g-0ubuntu0.11.04) ...

Current default time zone: 'America/Chicago'
Local time is now: Wed Apr 27 16:05:22 CDT 2011.
Universal Time is now: Wed Apr 27 21:05:22 UTC 2011.
Run 'dpkg-reconfigure tzdata' if you wish to change it.

# Now previous indicators are reset to America/Chicago instead of US/Central

ProblemType: Bug
DistroRelease: Ubuntu 11.04
Package: tzdata 2011g-0ubuntu0.11.04
ProcVersionSignature: Ubuntu 2.6.38-8.42-generic 2.6.38.2
Uname: Linux 2.6.38-8-generic x86_64
Architecture: amd64
Date: Wed Apr 27 15:54:33 2011
InstallationMedia: Ubuntu 10.10 "Maverick Meerkat" - Release amd64 (20101007)
PackageArchitecture: all
ProcEnviron:
 SHELL=/bin/bash
 PATH=(custom, user)
 LANG=en_US.UTF-8
 LANGUAGE=en_US:en
SourcePackage: tzdata
UpgradeStatus: Upgraded to natty on 2011-04-10 (17 days ago)