Manual pages: Standardize on AUTHORS as section title

Manual pages: Standardize on AUTHORS as section title

There is quite some value (in terms of readability and user
expectations) if consistent names are used for the sections
within manual pages. This patch is one of a series to bring
about this consistency.

In the Linux man-pages project, I long ago did away with the
AUTHOR(S) section, but I realize some projects like to keep this.

But, let's make sure that the section is consistently titled
across pages. Currently we have AUTHOR (47) or AUTHORS (41).
Let's standardize on the latter (which is also what is
suggested in man-pages(7)).

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>