basenc: A new program complementary to base64/base32

basenc: A new program complementary to base64/base32

Encodes/decodes data in various common formats:
base64,base64url,base32,base32,base16,base2,z85.

Discussed here:
https://lists.gnu.org/r/coreutils/2018-11/msg00014.html
https://lists.gnu.org/r/coreutils/2018-12/msg00019.html

* AUTHORS: Add basenc.
* README: Reference the new program.
* NEWS: Mention the new program.
* build-aux/gen-lists-of-programs.sh: Add basenc.
* doc/coreutils.texi: (basenc invocation): Document the new command.
* man/.gitignore: Ignore the generated man page.
* man/basenc.x: A new template, with few examples.
* man/local.mk: Reference the new man page.
* scripts/git-hooks/commit-msg: Allow basenc as program prefix.
* src/.gitignore: Ignore the new binary.
* src/basenc.c:
(usage): Mention new options.
(main): Handle new options.
(isbase*, base*_length, base*_encode, base*_decode_ctx): Implement new
encoding/decoding formats.
* src/local.mk: Add new program.
* tests/local.mk: Add new test.
* tests/misc/basenc.pl: New tests.
* tests/misc/help-version.sh (basenc_setup): use '--version' for default
invocation (basenc errors with no parameters).