Use posix format for dist tarballs

Use posix format for dist tarballs

Traditionally Automake has defaulted to the V7 tar format when
creating tarballs for distributions.  One of the many limitions
of this format is a 99 character maximum path + file name limit.
This can cause problems when adding new test cases to the ZTS
due to the depth of the sub-tree and descriptive test names.

This change switches the build system to the posix (aliased as
pax) tar format which conforms to the POSIX.1-2001 specification.
This format does not suffer from the V7 limitations, was designed
to be compatible, and will become the default format in future
versions of GNU tar.

https://www.gnu.org/software/tar/manual/html_chapter/tar_8.html

As part of this change the blockfiles directories which were
originally removed due to this limit have been readded.

Reviewed by: Tim Chase <tim@chase2k.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #7767