REORG: thread: move the thread init/affinity/stop to thread.c

REORG: thread: move the thread init/affinity/stop to thread.c

haproxy.c still has to deal with pthread-specific low-level stuff that
is OS-dependent. We should not have to deal with this there, and we do
not need to access pthread anywhere else.

Let's move these 3 functions to thread.c and keep empty inline ones for
when threads are disabled.