maint: simplify printing of md5sum file names

maint: simplify printing of md5sum file names

* src/md5sum.c (main): Add a comment as to why we continue
to escape names that do not have '\n' but do have '\\' chars.
(print_filename): Use the predetermined boolean to decide
whether to escape or not, so that in the common case we
can output the file name directly, rather than inspecting each char.
* tests/misc/md5sum.pl: Add case to show '\\' chars cause escaping.
* tests/misc/sha1sum.pl: Likewise.