b2sum: a new checksum utility with md5sum like interface

Note we don't support the --algorithm option of the
b2sum command in the external BLAKE2 project, as that
was deemed too confusing for users.
"BLAKE2b" was chosen as the default algorithm to use,
which is single threaded but performs well on 64 bit.

* src/blake2: CC0 source copied from external project.
* cfg.mk[VC_LIST_ALWAYS_EXCLUDE_REGEX]: Exclude blake2/
from syntax checks, make update-copyright, etc.
* src/local.mk: Reference the sources for b2sum,
and set the compilation flags.
* doc/coreutils.texi (b2sum invocation): Reference the
md5sum invocation node, and add descriptions of -l.
* tests/misc/b2sum.sh: Add new test.
* tests/local.mk: Reference new test.
* AUTHORS: Add new binary.
* README: Likewise.
* build-aux/gen-lists-of-programs.sh: Likewise.
* man/.gitignore: Likewise.
* scripts/git-hooks/commit-msg: Likewise.
* man/b2sum.x: New man page template.
* man/local.mk: Reference new template.
* src/.gitignore: Ignore new binaries.
* src/blake2/.gitignore: Ignore new build atrifacts.
* src/md5sum.c (usage): Describe the new -l option.
* NEWS: Mention the new program.

scripts/git-hooks/commit-msg

@@ -14,8 +14,8 @@ $editor = "vi" if $? != 0 or $editor =~ /^\s*\z/;
 # Keywords allowed before the colon on the first line of a commit message:
 # program names and a few general category names.
 my @valid = qw(
-    arch base32 base64 basename cat chcon chgrp chmod chown chroot cksum comm
-    cp csplit cut date dd df dir dircolors dirname du echo env expand
+    arch b2sum base32 base64 basename cat chcon chgrp chmod chown chroot cksum
+    comm cp csplit cut date dd df dir dircolors dirname du echo env expand
     expr factor false fmt fold groups head hostid hostname id install
     join kill link ln logname ls md5sum mkdir mkfifo mknod mktemp
     mv nice nl nohup nproc numfmt od paste pathchk pinky pr printenv printf