(In response to savannah bug #63574)
* doc/intern.texi: Document actual tar behaviour in regard to
missing end-of-file marker.
* doc/tar.texi: Rewrite the "warnings" section. Document
--warning=missing-zero-blocks
* src/common.h (WARN_MISSING_ZERO_BLOCKS): New constant.
(WARN_ALL): Include all warning bits.
* src/list.c (read_and): If EOF is reached without seeing end-of-file
blocks and the "missing-zero-blocks" warning is requested, warn about
the fact.
* src/warning.c: New warnings: "missing-zero-blocks", "verbose".
(warning_option): Change definition to reflect changes in common.h
* doc/Makefile.am (GENDOCS): Use the version from the tar repository.
(manual): Set the DISTRIN rendition.
(manual-rebuild): New goal.
* doc/README.manual: New file. Instructions for maintainers on how to
update web documentation.
* doc/gendocs.sh: A version from gnulib fixed as per
https://lists.gnu.org/archive/html/bug-gnulib/2021-03/msg00002.html.
* doc/gendocs_template: Updated version from gnulib.
* doc/intern.texi: Fix the use of UNREVISED.
* doc/tar.texi: Fix the use of GNUTAR.