Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

Implement strndup(3) using strnlen(3).

This makes the implementation a bit more consistent with strdup(3),
which uses strlen(3).

Import the strndup(3) function.
Copyright attribution is kept the same as in original NetBSD source.

Submitted by: Florian Smeets <flo kasimir com>
Obtained from: NetBSD
MFC after: 2 weeks