refreshcreds(p) has to be called in user-mode.

After debugging why the early debugger doesn't work we found
that in comparison to other architectures, refreshcreds(p)
is called in kernel-, instead of user-mode.  Therefore
curproc can be NULL at that time, and lead to a crash.

Reported to and discussed with guenther@openbsd

ok pedro@
Add comp set to maintenance make targets.

ok patrick@