Use ".pref" as file extension for preferences.d files

Packages with periods in the name will cause apt to think that everything to the right of the period is a file extension, and this causes apt to ignore those files and generate a notice:

N: Ignoring file 'postgresql-9.1' in directory '/etc/apt/preferences.d/' as it has an invalid filename extension
N: Ignoring file 'postgresql-client-9.1' in directory '/etc/apt/preferences.d/' as it has an invalid filename extension

Per apt_preferences(5) ".pref" is an acceptable extension and solves this issue.