Comment 2 for bug 854337

Confirmed. Same problem here, full log:

daniel@daydream:~$ LC_ALL=C sudo apt-get install xdm
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following NEW packages will be installed:
  xdm
0 upgraded, 1 newly installed, 0 to remove and 0 not upgraded.
Need to get 0 B/164 kB of archives.
After this operation, 926 kB of additional disk space will be used.
Preconfiguring packages ...
Selecting previously deselected package xdm.
(Reading database ... 172392 files and directories currently installed.)
Unpacking xdm (from .../xdm_1%3a1.1.10-3ubuntu1_amd64.deb) ...
Processing triggers for man-db ...
Processing triggers for ureadahead ...
Setting up xdm (1:1.1.10-3ubuntu1) ...
 Adding system startup for /etc/init.d/xdm ...
   /etc/rc0.d/K01xdm -> ../init.d/xdm
   /etc/rc1.d/K01xdm -> ../init.d/xdm
   /etc/rc6.d/K01xdm -> ../init.d/xdm
   /etc/rc2.d/S99xdm -> ../init.d/xdm
   /etc/rc3.d/S99xdm -> ../init.d/xdm
   /etc/rc4.d/S99xdm -> ../init.d/xdm
   /etc/rc5.d/S99xdm -> ../init.d/xdm
daniel@daydream:~$ LC_ALL=C sudo dpkg -P xdm
(Reading database ... 172427 files and directories currently installed.)
Removing xdm ...
xdm prerm warning: new default display manager has been selected; please be
   sure to run "dpkg-reconfigure lightdm" to ensure that it is configured
Purging configuration files for xdm ...
 Removing any system startup links for /etc/init.d/xdm ...
   /etc/rc0.d/K01xdm
   /etc/rc1.d/K01xdm
   /etc/rc2.d/S99xdm
   /etc/rc3.d/S99xdm
   /etc/rc4.d/S99xdm
   /etc/rc5.d/S99xdm
   /etc/rc6.d/K01xdm
Processing triggers for ureadahead ...
Processing triggers for man-db ...
daniel@daydream:~$ LC_ALL=C sudo dpkg-reconfigure lightdm
dpkg-maintscript-helper: warning: environment variable DPKG_MAINTSCRIPT_NAME missing
dpkg-maintscript-helper: warning: environment variable DPKG_MAINTSCRIPT_PACKAGE missing
daniel@daydream:~$