Don't retry a failing IRC nickname forever.

If our IRC nick is in use (because some other node thinks it has
the same address we think we have) don't fruitlessly try to reconnect
using that name forever. After three tries, give up and use a random
nick. Either we'll learn a new local address from IRC and switch
to that, or it was right and the other guy is advertising for us.

This avoids a pessimal case where a second testnet node behind
a nat is unable to get any peers because he can't get on IRC.