Issue 2548: Reading GNU sparse entries (#2558)

Issue 2548: Reading GNU sparse entries (#2558)

My attempt to fix #2404 just made the confusion between the size of the
extracted file and the size of the contents in the tar archive worse
than it was before.

@ferivoz in #2557 showed that the confusion stemmed from a point where
we were setting the size in the entry (which is by definition the size
of the file on disk) when we read the `GNU.sparse.size` and
`GNU.sparse.realsize` attributes (which might represent the size on disk
or in the archive) and then using that to determine whether to read the
value in ustar header (which represents the size of the data in the
archive).

The confusion stems from three issues:
* The GNU.sparse.* fields mean different things depending on the version
of GNU tar used.
* The regular Pax `size` field overrides the value in the ustar header,
but the GNU sparse size fields don't always do so.
* The previous libarchive code tried to reconcile different size
information as we went along, which is problematic because the order in
which this information appears can vary.

This PR makes one big structural change: We now have separate storage
for every different size field we might encounter. We now just store
these values and record which one we saw. Then at the end, when we have
all the information available at once, we can use this data to determine
the size on disk and the size in the archive.

A few key facts about GNU sparse formats:

* GNU legacy sparse format: Stored all the relevant info in an extension
of the ustar header.
* GNU pax 0.0 format: Used `GNU.sparse.size` to store the size on disk
* GNU pax 0.1 format: Used `GNU.sparse.size` to store the size on disk
* GNU pax 1.0 format: Used `GNU.sparse.realsize` to store the size on
disk; repurposed `GNU.sparse.size` to store the size in the archive, but
omitted this in favor of the ustar size field when that could be used.

And of course, some key precedence information:
* Pax `size` field always overrides the ustar header size field.
* GNU sparse size fields override it ONLY when they represent the size
of the data in the archive.

Resolves #2548

(cherry picked from commit 29fd918e1886abacca88864ad3676fa237ff21e2)