[util] Replace kutil with util

Now that kutil has no kernel-specific code in it anymore, it can
actually be linked to by anything, so I'm renaming it 'util'.

Also, I've tried to unify the way that the system libraries from
src/libraries are #included using <> instead of "".

Other small change: util::bip_buffer got a spinlock to guard against
state corruption.

src/kernel/objects/endpoint.h

@@ -2,7 +2,9 @@
 /// \file endpoint.h
 /// Definition of endpoint kobject types
 
-#include "j6/signals.h"
+#include <j6/signals.h>
+#include <util/vector.h>
+
 #include "objects/kobject.h"
 
 /// Endpoints are objects that enable synchronous message-passing IPC
@@ -64,5 +66,5 @@ private:
 
     j6_status_t do_message_copy(const thread_data &sender, thread_data &receiver);
 
-    kutil::vector<thread_data> m_blocked;
+    util::vector<thread_data> m_blocked;
 };