MINOR: init: extract the setup and end of threads to their own functions

MINOR: init: extract the setup and end of threads to their own functions

The startup code was still ugly with tons of unreadable nested ifdefs.
Let's just have one function to set up the extra threads and another one
to wait for their completion. The ifdefs are isolated into their own
functions now and are more readable, just like the end of main(), which
now uses the same statements to start thread 0 with and without threads.