md5sum, sha*sum: add --tag to output a format indicating the algorithm

md5sum, sha*sum: add --tag to output a format indicating the algorithm

The format used is the BSD traditional format which looks like:
MD5 (/dev/null) = d41d8cd98f00b204e9800998ecf8427e

* NEWS: Add new feature info.
* doc/coreutils.texi (md5sum invocation): Add detailed information
about the new --tag option.
* src/md5sum.c: Add the new --tag option for BSD-style output.
(bsd_split_3): Add ESCAPED_FILENAME parameter.
(print_filename): New function refactored from main().
(filename_unescape): New function refactored from split_3().
* tests/misc/md5sum-bsd: Add tests for the new feature.