pkttyagent: PolkitAgentTextListener leaves echo tty disabled if SIGINT/SIGTERM

If no password is typed into terminal during authentication raised by PolkitAgentTextListener, pkttyagent sends kill (it receives from systemctl/hostnamectl e.g.) without chance to restore echoing back on. This cannot be done in on_request() since it's run in a thread without guarantee the signal is distributed there.

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