digest: support windows format checksum files

digest: support windows format checksum files

Support checksum files with CRLF line endings,
which is a common gotcha for using --check on windows,
or with checksum files generated on windows.
Note we escape \r here to support the original coreutils format
(with file name at EOL), and file names with literal
\r characters as the last character of their name.

* src/digest.c (filename_unescape): Convert \\r -> \r.
(print_filename): Escape \r -> \\r.
(output_file): Detect \r chars in file names.
(digest_check): Ignore literal \r char at EOL.
* tests/misc/md5sum.pl: Add a test case.
* tests/misc/sha1sum.pl: Likewise.
* NEWS: Mention the improvement.