md5: fix *sum -c with missing files

If we fail to open one of the files in the file listing, we still need
to advance rec along with argv so that the checksum we're checking
against lines up with the file we're hashing.

Tests added both for the -c flag, as well as the -b and -t modes of
the *sum programs.

PR: 267722
Reviewed by: emaste (earlier version)
Approved by: re (cperciva)

(cherry picked from commit 8d78a0d331ec2c168efe6cb85bbc2da86e9a6124)
(cherry picked from commit 9eb694146a19bd0ee7439f6cab030a8d5c273977)