initiate PING if socket idles for too long

initiate PING if socket idles for too long

note that we only start tracking the monotonic timestamp of last
incoming socket activity in irc_poll(). because of that, we must
initialize recv_ts or we risk never sending the first PING when the
server doesn't send anything after the 001 numeric (e.g. znc with no
joined channels).

there isn't really a reason to configure ping interval/pong timeout, so
we just pick some values.