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.

libc/string/strlen.c

@@ -1,10 +1,11 @@
-#include <mango/libc/string.h>
+#include <kernel/libc/string.h>
 
-size_t strlen(const char *str) {
+size_t strlen(const char *str)
+{
 	size_t res = 0;
 	while (str[res]) {
 		res++;
 	}
-	
+
 	return res;
 }