xar: Add support for SHA256 and SHA512

xar: Add support for SHA256 and SHA512

Extend the xar reader and writer to understand the SHA256 (header
cksum_alg 3, 32 bytes, style "sha256") and SHA512 (cksum_alg 4, 64
bytes, style "sha512") checksum algorithms in addition to the existing
SHA1 and MD5.  These are the algorithm codes defined by the canonical
xar format, so archives written with them interoperate with Apple's xar,
pkgutil, and PackageKit.

Both the TOC ("toc-checksum") and per-file ("checksum") algorithms are
supported for writing.  New algorithm handling is guarded by
ARCHIVE_HAS_SHA256 / ARCHIVE_HAS_SHA512 so builds without those digests
are unaffected.  MAX_SUM_SIZE grows from 20 to 64 to hold a SHA512
digest.