md5sum: ensure a single status line per file

md5sum: ensure a single status line per file

* src/md5sum.c: Use the same file name escaping method used
when generating and checking checksums.  I.E. ensure a single line
per file by starting the line with '\' for any file name containing '\n'
and replacing those with "\\n".
* NEWS: Move the item from changes in behavior to improvements,
since this is no longer a backwards incompat change when
processing stdout status messages.
* tests/misc/md5sum.pl: Remove quotes from expected status output.
* tests/misc/sha1sum.pl: Likewise.