split: add the --additional-suffix option

split: add the --additional-suffix option

Add the --additional-suffix option, to append an
additional static suffix to output file names.

* src/split.c (next_file_name): Append suffix to output file names.
(main): Handle new --additional-suffix option.
* NEWS (New features): Mention it.
* doc/coreutils.texi (split invocation): Mention it.
* tests/split/additional-suffix: New file. Test --additional-suffix.
* tests/Makefile.am (TESTS): Add it.
Requested by Peng Yu, in bug 6554