ipn/ipnlocal: allow the ingress peer capability for unsigned peers (#20745)

0eb38dc2e (#20561) made peer capability resolution return nothing for
peers with UnsignedPeerAPIOnly set, so that a possibly malicious
control server can't grant capabilities to peers outside the tailnet
lock authority. But Tailscale Funnel ingress nodes are unsigned by
design, and control intentionally grants them
PeerCapabilityIngress, which the peerapi /v0/ingress handler requires.
The result was that every Funnel connection was rejected with a 403
"denied; no ingress cap".

Instead of denying all capabilities to unsigned peers, allowlist
PeerCapabilityIngress specifically. It only permits ingress requests
over the PeerAPI, which unsigned peers can already reach, and the
node only serves them for targets explicitly configured for Funnel.

The tsnet TestFunnel didn't catch the regression because its fake
ingress peer was a normal signed peer. Teach testcontrol to mark a
node as UnsignedPeerAPIOnly (excluding such nodes from traffic-
permitting filter rules, as real control does, so clients don't
discard the packet filter) and make TestFunnel use it so the test
now exercises the same capability checks as production Funnel
traffic.

Updates tailscale/corp#46053
Updates #20739


Change-Id: I3f6b8e2a94d1c07b5a2e9d84f16c30aa79e5d21b

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>