detect compressed status files on extension again

It handy to be able to point apt at reading a compressed dpkg/status
file in debugging cases, which worked pre-1.1 but somewhere down the
line in the massive refactoring. Restoring this behavior in a central
place for all realfile index files instead of just for the status file.

(This has no effect on index files acquired from an archive – those are
handled by different classes and support compressed files just fine)