md5: fix *sum -c with missing files

[kevans]

Summary

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

[emaste]

sbin/md5/tests/md5_test.sh

48–50

can just invoke sha256 a b c instead? or, perhaps even just create expected and sums with explicit content?