nbd-server: handle modern-style negotiation in a child process

Previously, the modern style negotiation was carried out in the root
server (listener) process before forking the actual client handler. This
made it possible for a malfunctioning or evil client to terminate the
root process simply by querying a non-existent export or aborting in the
middle of the negotation process (caused SIGPIPE in the server).

This commit moves the negotiation process to the child to keep the root
process up and running no matter what happens during the negotiation.

See http://sourceforge.net/mailarchive/message.php?msg_id=30410146
ndb-server: rename handle_connection() -> handle_oldstyle_connection()

The new name describes better the actual usage.

Remove also the fourth parameter, CLIENT*, which was always NULL.
nbd-server: refactor out common code
nbd-server: remove superfluous 2nd parameter from handle_oldstyle_connection()

A pointer to the server struct is passed anyways, the socket can be
accessed via that pointer.
nbd-server: const'd function parameter variable

Making the parameter variable const revealed a smelly assignment
statement which was deemed unnecessary and hence removed.