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