nbd-server: simplify and rename setup_serve() -> setup_oldstyle_server()

Previously, setup_serve() basically configured only oldstyle
connections and it just returned if oldstyle configuration was not
requested. This commit simplifies the setup_serve() by moving the
setup decision to the caller.
nbd-server: extract signal handler setup to a separate function

Do one thing and do it well.
nbd-server: split dousers() to setup_gid() and setup_uid()

Do one thing and do it well.
nbd-server: extract modern socket initialization from setup_servers()

Do one thing and do it well.
nbd-server: move child process table initialization to main()

Do one thing and do it well.
nbd-server: extract modern connection handling to handle_modern_connection()
nbd-server: extract oldstyle connection handling to handle_oldstyle_connection()
nbd-server: move connection accepting to handler functions
nbd-server: move client allocation to oldstyle specific section
nbd-server: extract child forking to a separate function
nbd-server: refactor common code
nbd-server: const'd local variables