BUF_strndup was calling strlen through BUF_strlcpy, and ended up reading
past the input if the input was not a C string.
Make it explicitly part of BUF_strndup's contract to never read more
than |siz| input bytes. This augments the standard strndup contract to
be safer.
The commit also adds a check for siz overflow and some brief documentation
for BUF_strndup().
Reviewed-by: Matt Caswell <matt@openssl.org>
(cherry picked from commit 110f7b37de)
