Kernel+LibC: Move errno definitions to Kernel/API/POSIX

This fixes at least half of our LibC includes in the kernel. The source
of truth for errno codes and their description strings now lives in
Kernel/API/POSIX/errno.h as an enumeration, which LibC includes.

Kernel/UserOrKernelBuffer.h

@@ -9,10 +9,10 @@
 
 #include <AK/Types.h>
 #include <AK/Userspace.h>
+#include <Kernel/API/POSIX/errno.h>
 #include <Kernel/Memory/MemoryManager.h>
 #include <Kernel/StdLib.h>
 #include <Kernel/UnixTypes.h>
-#include <LibC/errno_numbers.h>
 
 namespace Kernel {