md5sum, sha1sum, etc: accept new option: --strict

md5sum, sha1sum, etc: accept new option: --strict

Use this new option with --check when the input is expected to
consist solely of checksum lines.  With only --check, an invalid
line evokes a warning, but the program can still exit successfully.
With --strict, any invalid line makes the program exit non-zero.

* src/md5sum.c (strict, STRICT_OPTION): Declare/define.
(long_options): Add "strict".
(usage): Describe --strict.
(digest_check): Count improperly_formatted lines, too, and use
that number and the global "strict" to determine the return value.
(main): Handle STRICT_OPTION.
Reject --strict without --check.
* doc/coreutils.texi: Describe it.
* NEWS (New features): Mention it.