Bug#161097: textutils md5sum is happy with signed md5sum.txt files

[Lars Wirzenius <liw@iki.fi>]

Adam Heath:
> Signed files are not md5sum checksum files.

dpkg replaces the textutils md5sum with its own, and since they are
compatible, that's good. There are minor differences, and one of them is
that the textutils md5sum is happy with garbage lines in the input,
e.g., GPG signatures. The textutils md5sum info file even documents this
behavior, though in a somewhat understated manner.

It Would Be Nice If the dpkg md5sum would be compatible with the
texutils one in this regard as well. If nothing else, it would be good
for the error message to point out the actual problem (malformed input
line) instead of confusing the user with complaints about invented
filenames.