(read_and, list_archive): Simplify code.

(read_and, list_archive): Simplify code.
(read_header): Fix computation of signed checksums on machines where
char is unsigned.
Do not consider a block to be zero unless all its bytes are zero,
even the checksum bytes.  Do not attempt to parse the checksum of
a zero block.  Fix memory leak with long names and links.
(from_chars): Accommodate a buggy tar that outputs leading NUL
if the previous field overflows.
(skip_extended_headers): Don't assume find_next_block yields nonnull.