Added more forgiving CRC checking logic when reading tar files

Added more forgiving CRC checking logic when reading tar files

Tim Kientzle posted the following to the libtar mailing list regarding
the earlier CRC bug:

	This exact bug has reappeared many times in different
	tar implementations.  Here are two suggestions:

	* The standards specify that the checksum is
	calculated using unsigned characters.

	* The most robust tar readers will compute the
	checksum both ways (with unsigned characters and
	with signed characters) and accept the header
	if either version matches.

https://lists.feep.net:8080/pipermail/libtar/2012-July/000327.html

He gave the following links:

	http://pubs.opengroup.org/onlinepubs/9699919799/

	"pax" is under "Shell & Utilities" => "Utilities"; here's
	a direct link:

	http://pubs.opengroup.org/onlinepubs/9699919799/utilities/pax.html
	https://github.com/libarchive/libarchive/wiki/ManPageTar5

This commit turns libtar into a "more robust tar reader."