libfetch: Use memcpy in place of an odd strncpy.

[jhb]

Summary

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.

[jhb]

Note to self: fix spelling of "However" in log message.

[des]

Looks like a job for strlcpy()...

[imp]

This looks good to me. I disagree strlcpy() is right here. This is a fixed length copy w/o a NUL termination.