[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/page_table.h

@@ -3,9 +3,9 @@
 /// Helper structures for dealing with page tables.
 
 #include <stdint.h>
-#include "enum_bitfields.h"
-#include "kernel_memory.h"
-#include "kutil/spinlock.h"
+#include <util/spinlock.h>
+#include <enum_bitfields.h>
+#include <kernel_memory.h>
 
 struct free_page_header;
 
@@ -142,7 +142,7 @@ struct page_table
 
     static free_page_header *s_page_cache; ///< Cache of free pages to use for tables
     static size_t s_cache_count;           ///< Number of pages in s_page_cache
-    static kutil::spinlock s_lock;         ///< Lock for shared page cache
+    static util::spinlock s_lock;          ///< Lock for shared page cache
 
     /// Get an entry in the page table as a page_table pointer
     /// \arg i     Index of the entry in this page table