r3356: in the standard process model we need to make sure we close all

r3356: in the standard process model we need to make sure we close all
listening sockets after the fork to prevent the child still listening
on incoming requests.

I have also added an optimisation where we use dup()/close() to lower
the file descriptor number of the new socket to the lowest possible
after closing our listening sockets. This keeps the max fd num passed
to select() low, which makes a difference to the speed of select().
(This used to be commit f2a9bbc317ba86ebe87c3ca27a3a0192de91014d)