logfile writing

Hiya,

I've written some code to push logfile writing into an external process,
freeing up the main squid process from the potentially blocking
stdio writes.

This code isn't ready to be merged into squid-2.6. Its meant more as a
prototype. I haven't even reviewed it for 'correctness' besides running
it under a reasonable load (200req/sec) for half a day.

the todo list:

* finish making stuff configurable
* rework the socket IO scheduling code to try and hold onto a buffer until
  its full or 1 second has passed. It might not really matter but it'll be
  interesting to profile it at very high (>1500/sec) request rates.
* figure out what to do about rotate (as it stands, this code only re-opens
  the logfiles and therefore pushes logfile maintainence into an external
  script.)

It has the cute side-effect of allowing people to finally write plugins to
easily throw squid logs into an external program - eg mysql logging, or
something to log over TCP to a central logging server, etc.

Adrian