Parse tar headers incrementally (#2127)

Parse tar headers incrementally (#2127)

This rebuilds the tar reader to parse all header data incrementally as
it appears in the stream.

This definitively fixes a longstanding issue with unsupported pax
attributes. Libarchive must limit the amount of data that it reads into
memory, and this has caused problems with large unknown attributes. By
scanning iteratively, we can instead identify an attribute by name and
then decide whether to read it into memory or whether to skip it without
reading.

This design also allows us to vary our sanity limits for different pax
attributes (e.g., an attribute that is a single number can be limited to
a few dozen bytes while an attribute holding an ACL is allowed to be a
few hundred kilobytes). This allows us to be a little more resistant to
malicious archives that might try to force allocation of very large
amounts of memory, though there is still work to be done here.

This includes a number of changes to archive_entry processing to allow
us to consistently keep the _first_ appearance of any given value
instead of the original architecture that recursively cached data in
memory in order to effectively process all the data from back-to-front.

Resolves #1855
Resolves #1939