[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/gdt.cpp

@@ -1,7 +1,7 @@
 #include <stdint.h>
 #include <string.h>
 
-#include "kutil/no_construct.h"
+#include <util/no_construct.h>
 
 #include "assert.h"
 #include "console.h"
@@ -22,7 +22,7 @@ static constexpr uint8_t tss_index       = 6; // Note that this takes TWO GDT en
 // The BSP's GDT is initialized _before_ global constructors are called,
 // so we don't want it to have a global constructor, lest it overwrite
 // the previous initialization.
-static kutil::no_construct<GDT> __g_bsp_gdt_storage;
+static util::no_construct<GDT> __g_bsp_gdt_storage;
 GDT &g_bsp_gdt = __g_bsp_gdt_storage.value;