kernel: separate headers into kernel and user headers

all kernel headers have been moved from include/mango to include/kernel
and include definitions that are only relevant to kernel-space.

any definitions that are relevant to both kernel- and user-space
(i.e. type definitions, syscall IDs) have been moved to
include/mango within libmango.

kernel/arg.c

@@ -1,6 +1,6 @@
-#include <mango/arg.h>
-#include <mango/libc/string.h>
-#include <mango/libc/ctype.h>
+#include <kernel/arg.h>
+#include <kernel/libc/string.h>
+#include <kernel/libc/ctype.h>
 
 static char g_cmdline[CMDLINE_MAX + 1] = {0};