b2sum: a new checksum utility with md5sum like interface

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.