nbd-server: fix export socket initialization
nbd-server: simplify setup_serve()
nbd-server: extract signal handler setup to a separate function
nbd-server: split dousers() to setup_gid() and setup_uid()
nbd-server: split setup_servers() to setup_export_sockets() and setup_modern_socket()
nbd-server: move child process table initialization to main()
nbd-server: rename setup_serve() to setup_export_socket()
nbd-server: extract modern connection handling to handle_modern_connection()
nbd-server: handle_oldstyle_connection()
nbd-server: move accepting connections to handle functions
nbd-server: move client allocation to old-style specific section
nbd-server: move child forking to upper layer
nbd-server: rename serveloop() -> listen_forever()
nbd-server: rename handle_ -fuctions to accept_-functions