route: avoid overlapping strcpy

Passing overlapping buffers to strcpy yields an undefined result, so
let's avoid it. The copy doesn't really need to happen anyways, we can
just point to the domain part of the hostname.

This was discovered with _FORTIFY_SOURCE.

Sponsored by: Klara, Inc.
Sponsored by: Stormshield
Reviewed by: allanjude, emaste, imp, melifaro (all previous version)
Differential Revision: https://reviews.freebsd.org/D32615