Move threading to c++11 primitives (mostly).

This moves all threading code in Bro from pthreads to the c++11
primitives, which make for shorter, easier to use, and less error-prone
code.

pthreads is still used in 2 places in Bro currently. BasicThread uses
two bits of functionality that are not available using the c++ API
(setting thread names & setting signal masks). Since all c++
implementations that I am aware of still use an underlying pthreads
implementation, we just use native_handle to access the underlying
pthreads implementation for these cases. I do not expect this to lead to
problems in the forseable future. If we ever encounter a platform where
a different thread architecture is used, we might have to change that
around.

This code is guarded by static_asserts, so we will notice if a platform
uses a different implementation.

sqlite also uses pthreads directly.

src/threading/MsgThread.h

@@ -2,8 +2,6 @@
 #ifndef THREADING_MSGTHREAD_H
 #define THREADING_MSGTHREAD_H
 
-#include <pthread.h>
-
 #include "DebugLogger.h"
 
 #include "BasicThread.h"