Kernel: Fix most of mutex + block race conditions

All block functions now take an optional mutex parameter that is
atomically unlocked instead of having the user unlock it before hand.
This prevents a ton of race conditions everywhere in the code!

kernel/include/kernel/Process.h

@@ -331,7 +331,6 @@ namespace Kernel
 
 		bool m_is_userspace { false };
 
-		SpinLock m_child_exit_lock;
 		BAN::Vector<ChildExitStatus> m_child_exit_statuses;
 		ThreadBlocker m_child_exit_blocker;