patch: half closed stuff

Hi,

Here's a patch which fixes the FTP half-closed crash I've seen.
It does this:

* exports a commIfHalfClosed(fd) routine..
* .. which I then use in ConnStateData::readSomeData() to
  make 100% sure I'm not reading any data from that fd..
* Started calling AbortChecker::doIOLoop() once a second
  from an event.

The background with this: my FTP connections were being requested
with a Keepalive header and so, even though the read side had
marked the FD as half-closed, the write side completed, discovered
there's a keepalive and then attempted to read some more data.

Comments?

Adrian