KTar: use unsigned arithmetic when calculating checksum of tar header record.

KTar: use unsigned arithmetic when calculating checksum of tar header record.

According to the ustar specification, implementations must use unsigned
arithmetic when calculating checksum field of the tar header record. KTar prior
to this patch used signed arithmetic for checksum calculation when writing an
archive. The patch fixes this.

However, there are more broken tar implementations out there (including former
KTar itself) so the patch also makes KTar to verify checksums using both
unsigned and signed arithmetic when reading archives. If either of checksums
matches, archive is accepted.

Implemented in ktar_header_checksum_fix.diff patch.