Do not fail in upgrade mode (-u) when configuration files have been modified

Custom configuration files might be present and upgrading
packages might fail then:

| Setting up sudo (1.8.9p5-1) ...
|
| Configuration file '/etc/sudoers'
|  ==> File on system created by you or by a script.
|  ==> File also in package provided by package maintainer.
|    What would you like to do about it ?  Your options are:
|     Y or I  : install the package maintainer's version
|     N or O  : keep your currently-installed version
|       D     : show the differences between the versions
|       Z     : start a shell to examine the situation
|  The default action is to keep your current version.
| *** sudoers (Y/I/N/O/D/Z) [default=N] ? dpkg: error processing package sudo (--configure):