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

src/threads.lua

@@ -40,9 +40,12 @@ function thread_mt:join()
 	return self.thread:join()
 end
 
-local bootstrap_code = [[
+local bootstrap_pre = [[
 local action, action_arg, parent_ctx = ...
 local func
+]]
+
+local bootstrap_post = [[
 
 -- copy parent ZeroMQ context to this child thread.
 local zmq = require"zmq"
@@ -69,6 +72,8 @@ end
 return func(select(4, ...))
 ]]
 
+local bootstrap_code = bootstrap_pre..bootstrap_post
+
 local function new_thread(ctx, action, action_arg, ...)
 	-- convert ZMQ_Ctx to lightuserdata.
 	if ctx then
@@ -82,6 +87,10 @@ end
 
 module(...)
 
+function set_bootstrap_prelude(code)
+   bootstrap_code = bootstrap_pre .. code .. bootstrap_post
+end
+
 function runfile(ctx, file, ...)
 	return new_thread(ctx, 'runfile', file, ...)
 end