Let /etc/installurl default to cdn.openbsd.org if it doesn't exist and no

Let /etc/installurl default to cdn.openbsd.org if it doesn't exist and no
official mirror was used. This way, people doing installs without network
access also get working pkg_add and syspatch experience out of the box.

Idea from tj
ok halex, job (who made it all possible), deraadt