checklocks: synchronize UDP receive state

Packet delivery reads frozen under rcvMu, but freeze and thaw update
it under mu. Restore also writes it without rcvMu. Use the receive
mutex for these writes and reuse thaw after a successful restore,
retaining the existing mu-to-rcvMu ordering and failure cleanup.

Guard the receive queue fields and propagate non-reentrancy contracts
through their concrete callers. Extend the existing bound-endpoint
test to resume the endpoint alongside real packet delivery and check
that the payload is received.

Assisted-by: Codex