field when computing the length of the gzip header.

Thanks to Dag-Erling for pointing me to the OpenSSH tarballs,
which are the first files I've seen that actually used this field.

Not an issue for FreeBSD, since the base system has the necessary libraries.
Since all decompressors are always available now, we can unconditionally
enable them in archive_read_support_compression_all().

use external "gunzip" instead.  With this in place, we can unconditionally
enable gzip read support.

This implements the new generic options framework that provides a way
to override format- and compression-specific parameters.

to fully take advantage of the new peek/consume I/O support.
In particular, this now properly handles concatenated gzip streams.

This is the last phase of the "big decompression refactor" that
puts a lazy reblocking layer between each pair of read filters.
I've also changed the terminology for this area---the two kinds
of objects are now called "read filters" and "read filter bidders"---and
moved ownership of these objects to the archive_read core.

This greatly simplifies implementing new read filters, which
can now use peek/consume I/O semantics both for bidding (arbitrary
look-ahead!) and for reading streams (look-ahead simplifies handling
concatenated streams, for instance).

The first merge here is the overhaul proper; the remainder are small
fixes to correct errors in the initial implementation.

end of the compressed stream.  This is desirable behavior,
but the implementation here is very broken and causes strange
problems, so disable it for now.

Thanks to Simon L. Nielsen for reporting this problem.

This is an attempt to eliminate a lot of redundant
code from the read ("decompression") filters by
changing them to juggle arbitrary-sized blocks
and consolidate reblocking code at a single point
in archive_read.c.

Along the way, I've changed the internal read/consume
API used by the format handlers to a slightly
different style originally suggested by des@.  It
does seem to simplify a lot of common cases.

The most dramatic change is, of course, to
archive_read_support_compression_none(), which
has just evaporated into a no-op as the blocking
code this used to hold has all been moved up
a level.

There's at least one more big round of refactoring
yet to come before the individual filters are as
straightforward as I think they should be...

  * "compression_program" support uses an external program
  * Portability: no longer uses "struct stat" as a primary
    data interchange structure internally
  * Part of the above: refactor archive_entry to separate
    out copy_stat() and stat() functions
  * More complete tests for archive_entry
  * Finish archive_entry_clone()
  * Isolate major()/minor()/makedev() in archive_entry; remove
    these from everywhere else.
  * Bug fix: properly handle decompression look-ahead at end-of-data
  * Bug fixes to 'ar' support
  * Fix memory leak in ZIP reader
  * Portability: better timegm() emulation in iso9660 reader
  * New write_disk flags to suppress auto dir creation and not
    overwrite newer files (for future cpio front-end)
  * Simplify trailing-'/' fixup when writing tar and pax
  * Test enhancements:  fix various compiler warnings, improve
    portability, add lots of new tests.
  * Documentation: document new functions, first draft of
    libarchive_internals.3

MFC after: 14 days
Thanks to: Joerg Sonnenberger (compression_program)
Thanks to: Kai Wang (ar)
Thanks to: Colin Percival (many small fixes)
Thanks to: Many others who sent me various patches and problem reports.