The iteration over all pcbs is possible without the global list. The
newborn inpcbs are put on a global list of unconnected inpcbs, then after
connect(2) or bind(2) they move to respective hash slot list.

This adds a bit of complexity to inp_next(), but the storage scheme is
actually simplified.

One potential problem before this change was that a couple of pcbs fall
into the same hash slot and are linked A->B there, but they also sit next
to each other in the global list, linked as B->A. This can deadlock of
course. The problem was never observed in the wild, but I was able to
instrument it with lots of effort: just few pcbs in the system, hash size
reduced down to 2 and a lot of repetitive calls into two kinds of
iterators.

However the main motivation is not the above problem, but make a step
towards splitting the big hash lock into per-slot locks.