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)
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D37374