* more work for the DefaultAptSources spec

* more work for the DefaultAptSources spec
apt-pkg/acquire-item.h:
- add new pkgAcquire::Item::StatTransientNetworkError status

apt-pkg/acquire-item.cc:
- if we get a StatTransientNetworkError use old sigfile and indexfiles

apt-pkg/acquire-worker.cc:
- set StatTransientNetworkError on "Timeout", "TmpResolveFailure", "ConnectionRefused"

cmdline/apt-get.cc:
- handle a StatTransientNetworkError different than a normal error (warning instead of error)