Closes the DNS rebinding hole, which the origin guard structurally cannot:
under rebinding the attacker's page keeps its own name, so Origin and Host
agree with each other and the origin guard trusts them both, while the
address behind that name is the victim's Spoolman instance.
The default allows everything a stranger cannot point at someone else's
machine -- any IP literal, any single-label name, and the non-registrable
suffixes -- because what the attack needs is a registrable public domain.
That leaves a reverse proxy on a real domain, which cannot be told apart
from the attack and so must declare itself via SPOOLMAN_ALLOWED_HOSTS.