Encode filenames in the VIS_CSTYLE | VIS_OCTAL style regardless of
output mode. When reading filenames from a checksum file, attempt to
decode them, and use the decoded name unless the decoded name does not
exist but the undecoded one does.
This breaks compatibility with GNU coreutils, which unfortunately uses
a non-reversible encoding when outputting filenames containing
non-printable characters.
While here, drop a sentence about preimage attacks against MD5 and SHA1
from the manual page, as I no longer trust it to be true.
MFC after: 1 week