fixup foldmarkers in acquire-item.cc

Git-Dch: Ignore
ensure partial dirs are 0700 and owned by _apt:root

Reworks the API involved in creating and setting up the fetcher to be a
bit more pleasent to look at and work with as e.g. an empty string for
no lock isn't very nice. With the lock we can also stop creating all our
partial directories "just in case". This way we can also be a bit more
aggressive with the partial directory itself as with a lock, we know we
will gone need it.
use _apt:root only for partial directories

Using a different user for calling methods is intended to protect us
from methods running amok (via remotely exploited bugs) by limiting what
can be done by them. By using root:root for the final directories and
just have the files in partial writeable by the methods we enhance this
in sofar as a method can't modify already verified data in its parent
directory anymore.

As a side effect, this also clears most of the problems you could have
if the final directories are shared without user-sharing or if these
directories disappear as they are now again root owned and only the
partial directories contain _apt owned files (usually none if apt isn't
running) and the directory itself is autocreated with the right
permissions.
display errortext for all Err as well as Ign logs

consistently using Item::Failed in all specializec classes helps setting
up some information bits otherwise unset, so some errors had an empty
reason as an error. Ign is upgraded to display the error message we
ignored to further help in understanding what happens.
UpperCase some functions for consistency

changeOwnerAndPermissionOfFile->ChangeOwnerAndPermissionOfFile
preparePartialFile->GetPartialFileName
preparePartialFileFromURI->GetPartialFileNameFromURI

Git-Dch: ignore