build: fix dependencies of man/sha*sum.1

Although the above man pages depend on src/md5sum.c as a shared
source, the build of the man pages directly requires their own
executables to exist.

* man/local.mk (man/sha1sum.1): Change the dependency from
'src/md5sum' to 'src/sha1sum'.
(man/sha224sum.1): s/md5sum/sha224sum/
(man/sha256sum.1): s/md5sum/sha256sum/
(man/sha384sum.1): s/md5sum/sha384sum/
(man/sha512sum.1): s/md5sum/sha512sum/

Reported by Pádraig Brady in
http://lists.gnu.org/archive/html/coreutils/2013-11/msg00006.html
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.
md5sum, sha*sum: improve help for --check related options

* src/md5sum.c (usage): s/three/four/ in the message pertaining
to the --check related options.  Also clarify that --strict
is just significant for the formatting of the checksum lines.
Also since we're changing both strings, move the --strict description
in with the description of the other options and order alphabetically.
* THANKS.in: Added reporter: Daniel Mach