IPC patch

Ok,
        my proposed patch is up on www.squid-cache.org/~robertc/

Please note: more can be done, but I need to flush what I've done to
date, to prevent plot lossage.

SO: Henrik/Adrian, either say this is ok with you, or please give some
feedback on what feels wrong, and I'll tweak it to match.

The major changes are splitting all the 'if type == X' blocks into sub
classes of IPCInstance, and also creating the separate spawn class.

Anyway, hope to hear back soon,
Rob