Add additional code to llthreads bootstrap
Add additional code to llthreads bootstrap

This patch lets you add additional code to the beginning of the
bootstrap function executed within each new llthread.  This is needed,
for instance, if you've installed lua-zmq via luarocks; in this case,
you need to require "luarocks.loader" before you can require "zmq".  So
you can do something like:

    if package.loaded["luarocks.loader"] then
       zmq.threads.set_bootstrap_prelude([[require "luarocks.loader"]])
    end

azukiapp/lua-zmq,Neopallium/lua-zmq