Add in the client-side support for using POSIX shm for the response
buffer.

It's still memory inefficient (as there's now two copies of the incoming
data sitting in the client side) but it's bloating out the client side,
_not_ the server-side memory requirements.
This is the first cut of the server side SHM sysctl response stuff.

This allows the response buffer to be passed in via POSIX SHM.
It avoids having to allocate a temporary buffer to write a large
(potentially > 50 megabytes in the case of the pcbinfo responses)
and serialise it out via a socket - and then have the receiver have
to also allocate a large temporary buffer.