libfetch: Use memcpy in place of an odd strncpy.

libfetch: Use memcpy in place of an odd strncpy.

The length passed to strncpy is the length of the source string, not
the destination buffer.  This triggers a non-fatal warning in GCC 12.
Hoewver, the code is also odd.  It is really just a memcpy of the
string without its nul terminator.  For that use case, memcpy is
clearer.

Reviewed by:	imp, emaste
Differential Revision:	https://reviews.freebsd.org/D36824

(cherry picked from commit 611cf392672cf7aa52a593412fb2537546a7d6a4)