Update ZeroMQ support to version 4.0.4.

README: Update dependencies.
configure.ac: Check for zmq_curve_keypair() to silence warning.
doc/mud.zmq.text: Describe new bindings.
doc/mudcore.texi: Get rid of docs -zmq-io-threads.
src/lua_zmq.c: Update binding.
src/mudcore.c: Use zmq_ctx_new() instead of zmq_init().
src/options.c: Remove -zmq-io-threads.
src/options.h: Remove options_zmq_io_threads().
Randomise the order descriptors get their commands and add anti-flood.

A misbehaving client could flood the server with data, and cause it to
lock up as it tried to flush the overflow messages from the output
queue.

TODO: Remove old entries.
doc/mud.descriptor.texi: Describe random round-robin behaviour.
src/descriptor.c: Randomise descriptor order and flood-protect.
Use ntohs instead of htons when reading NAWS values.

src/descriptor.c: Fix.
Rename descriptor_new_fd() to descriptor_new().

src/descriptor.c: Fix.
src/descriptor.h: Fix.
src/io.c: Fix.
Clarify instance variables on mudcore.descriptor objects.

doc/mud.descriptor.texi: Fix.