commloops revisited

I'd like to break the comm code out into per-commtype files; much like
squid-3.

Here's my first cut. They compile and run; I haven't done much
testing on them. There's plenty more tidying up which can be done
after this is in - mostly involving tidying up commSetSelect() and
making the commDeferFD/commResumeFD calls part of the net API rather
than just for epoll.

I'd like to get this into squid-2.6 now so I or someone else can bring
across kqueue/solaris epoll before 2.6 is released.

I also discovered some recent change to the select() code stopped it
compiling; I've fixed it with what a cursory glance shows should be
right. I'd appreciate someone verifying this.

Adrian