Modernize Mutex

- Based on C++11's `mutex`
- No more need to allocate-deallocate or check for null
- No pointer anymore, just a member variable
- Platform-specific implementations no longer needed
- Simpler for `NO_THREADS`
- `BinaryMutex` added for special cases as the non-recursive version
- `MutexLock` now takes a reference. At this point the cases of null `Mutex`es are rare. If you ever need that, just don't use `MutexLock`.
- `ScopedMutexLock` is dropped and replaced by `MutexLock`, because they were pretty much the same.

drivers/unix/os_unix.cpp

@@ -36,7 +36,6 @@
 #include "core/project_settings.h"
 #include "drivers/unix/dir_access_unix.h"
 #include "drivers/unix/file_access_unix.h"
-#include "drivers/unix/mutex_posix.h"
 #include "drivers/unix/net_socket_posix.h"
 #include "drivers/unix/semaphore_posix.h"
 #include "drivers/unix/thread_posix.h"
@@ -63,6 +62,7 @@
 #include <string.h>
 #include <sys/time.h>
 #include <sys/wait.h>
+#include <time.h>
 #include <unistd.h>
 
 /// Clock Setup function (used by get_ticks_usec)
@@ -122,13 +122,11 @@ void OS_Unix::initialize_core() {
 #ifdef NO_THREADS
 	ThreadDummy::make_default();
 	SemaphoreDummy::make_default();
-	MutexDummy::make_default();
 #else
 	ThreadPosix::make_default();
 #if !defined(OSX_ENABLED) && !defined(IPHONE_ENABLED)
 	SemaphorePosix::make_default();
 #endif
-	MutexPosix::make_default();
 #endif
 	FileAccess::make_default<FileAccessUnix>(FileAccess::ACCESS_RESOURCES);
 	FileAccess::make_default<FileAccessUnix>(FileAccess::ACCESS_USERDATA);
@@ -307,13 +305,9 @@ Error OS_Unix::execute(const String &p_path, const List<String> &p_arguments, bo
 
 		while (fgets(buf, 65535, f)) {
 
-			if (p_pipe_mutex) {
-				p_pipe_mutex->lock();
-			}
+			p_pipe_mutex->lock();
 			(*r_pipe) += String::utf8(buf);
-			if (p_pipe_mutex) {
-				p_pipe_mutex->unlock();
-			}
+			p_pipe_mutex->unlock();
 		}
 		int rv = pclose(f);
 		if (r_exitcode)