Remove threads.h and replace its definitions with their C++11 equivalents (#5957)

This also changes threadProc's signature, since C++11 supports arbitrary
thread function signatures.

src/log.h

@@ -24,8 +24,8 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #include <queue>
 #include <string>
 #include <fstream>
+#include <thread>
 #include <mutex>
-#include "threads.h"
 #include "irrlichttypes.h"
 
 class ILogOutput;
@@ -79,7 +79,7 @@ private:
 	// written to when one thread has access currently).
 	// Works on all known architectures (x86, ARM, MIPS).
 	volatile bool m_silenced_levels[LL_MAX];
-	std::map<threadid_t, std::string> m_thread_names;
+	std::map<std::thread::id, std::string> m_thread_names;
 	mutable std::mutex m_mutex;
 	bool m_trace_enabled;
 };