Revamp semantics for `missing help2man' and manpage distribution.

Revamp semantics for `missing help2man' and manpage distribution.

Previously, `missing help2man' would create a missing man page
containing an error message, and exit 1.  This does not play
well with `make': the next run will see this particular man page
as being up to date, and will only error out on the next
generated man page, if any; repeat until all pages are done.
This patch changes `missing' to exit successfully in this case,
but `make dist' will ensure that no such man pages are packaged.

* lib/missing: Exit successfully even if we create a replacement
page due to missing help2man.
* automake.in (make_paragraphs): Define %HAVE-MANS% to be true
if this makefile deals with man pages.
* lib/am/distdir.am (distdir): If %INSTALL-MAN% and %HAVE-MANS%,
check that no man page in $(MANS) contains the replacement text
from `missing'.
* tests/man4.test: New test.
* tests/Makefile.am: Update.
* NEWS: Reorder a bit, update.
* THANKS: Update.
Report by Werner Lemberg and Karl Berry.

Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>