md5sum,sha*sum: add --ignore-missing for checking a subset of files

md5sum,sha*sum: add --ignore-missing for checking a subset of files

* doc/coreutils.texi (md5sum invocation): Document the new option.
* src/md5sum.c (digest_file): Return an empty digest to indicate
a missing file.
(digest_check): Don't fail or output status given an empty checksum.
(usage): Document the new option.
(main): Process and validate the new option.
* tests/misc/md5sum.pl: Add new test cases.
* NEWS: Mention the new feature.
Fixes http://bugs.gnu.org/15604