Add wcsnlen(3), wcslen(3) with a max len argument

Missing function hit by fcambus@ some time ago. ok millert@

Use the fancy .In macro for includes. From Jan Klemkow. ok jmc schwarze

update HISTORY and SEE ALSO for all of string(3), together with various
smaller improvements; jmc@ and nicm@ both ask me to get this in because
further tweaking will be easier in-tree

Instead of documenting all the wide string functions in wmemchr(3), add
individual pages (based on the existing string man pages). By Tim van
der Molen (tbvdm at xs4all dot nl) after a suggestion by

Instead of documenting all the wide string functions in wmemchr(3), add
individual pages (based on the existing string man pages). By Tim van
der Molen (tbvdm at xs4all dot nl) after a suggestion by millert@.

ok deraadt

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