Fix test_tarfile failures on Alpha (Tru64). The problem was caused in r61449

Fix test_tarfile failures on Alpha (Tru64).  The problem was caused in r61449
which made the return value signed.  On the Alpha that also lost data
since sizeof(int) != sizeof(long) and apparently adler32/crc32 return
64 bits of data.  This change keeps the signedness and continues to store the
data in a long rather than an int as was the case before r61449.