Fix the newly introduced method GetListOfFilesInDir to not accept every

Fix the newly introduced method GetListOfFilesInDir to not accept every
file if no extension is enforced (= restore old behaviour). (Closes: #565213)

This commit includes also:
* apt-pkg/policy.cc:
  - accept also partfiles with "pref" file extension as valid
* apt-pkg/contrib/configuration.cc:
  - accept also partfiles with "conf" file extension as valid
* doc/apt.conf.5.xml:
  - reorder description and split out syntax
  - add partfile name convention (Closes: #558348)
* doc/apt_preferences.conf.5.xml:
  - describe partfile name convention also here

And a lovely test application of course.