Update the required version of ZeroMQ checked by configure.ac.

configure.ac: Do it.
Move API function from mud global table to mudcore global table.

boot.lua: Update example.
doc/Makefile.am: Change filenames.
doc/mud.args.texi -> doc/mudcore.args.texi: Rename and update.
doc/mud.descriptor.texi -> doc/mudcore.descriptor.texi: Ditto.
doc/mud.log.texi -> doc/mudcore.log.texi: Ditto.
doc/mud.log.texi -> doc/mudcore.log.texi: Ditto.
doc/mud.zmq.texi -> doc/mudcore.zmq.texi: Ditto.
doc/mudcore.texi: Update references.
src/descriptor.c: Update.
src/lua_api.c: Ditto.
src/lua_args.c: Ditto.
src/lua_args.h: Ditto.
src/lua_descriptor.c: Ditto.
src/lua_descriptor.h: Ditto.
src/lua_log.c: Ditto.
src/lua_log.h: Ditto.
src/lua_timer.c: Ditto.
src/lua_timer.h: Ditto.
src/lua_zmq.c: Ditto.
src/lua_zmq.h: Ditto.
Use luaL_newlib() to create library tables.

src/lua_api.c: Do it.
src/lua_log.c: Ditto.
src/lua_timer.c: Ditto.
src/lua_zmq.c: Ditto.