Move the isc_random API initialization to the thread_local variable

Instead of writing complicated wrappers for every thread, move the
initialization back to isc_random unit and check whether the random seed
was initialized with a thread_local variable.

Ensure that isc_entropy_get() returns a non-zero seed.

This avoids problems with thread sanitizer tests getting stuck in an
infinite loop.

lib/isc/loop.c

@@ -42,7 +42,6 @@
 #include "async_p.h"
 #include "job_p.h"
 #include "loop_p.h"
-#include "random_p.h"
 
 /**
  * Private