shm/dpool: replace first_chunk with a list_head
shm/dpool: replace first_chunk with a list_head

Use a real "head" pointer, preparing to use boost::intrusive::list.

CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy