Fix PING-PONG handling when processing backlog in read buffers

Fix PING-PONG handling when processing backlog in read buffers

Prior to this commit, the PONG wasn't registered correctly, becauuse the
"last ping" time was set to time(NULL), which could be bigger than the
"last data" time stamp, for example when handling the read buffer took
more than 1 second -- and this resulted in the PONG time out kicking in
effectively disconnecting a newly linked server for example, because
ngIRCd thought it was still waiting for a PONG: last data < last ping.

Now the "last ping" value has three possible values:

    0: new connection, no PING, no PONG so far.
    1: got a PONG, no longer waiting for a PONG.
  <t>: time stamp of last sent out PING command.