MINOR: threads: make tg point to the current thread's group

A the "tg" thread-local variable now always points to the current
thread group. It's pre-initializd to the first one during boot and is
set to point to the thread's one by ha_set_tid(). This last one takes
care of checking whether the thread group was assigned or not because
it may be called during boot before threads are initialized.
1 file changed