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/memset.c

@@ -2,12 +2,12 @@
 
 void *memset(void *str, int c, size_t n)
 {
-    unsigned char val = (unsigned char)c;
-    unsigned char *buf = str;
+	unsigned char val = (unsigned char)c;
+	unsigned char *buf = str;
 
-    for (size_t i = 0; i < n; i++) {
-        buf[i] = val;
-    }
+	for (size_t i = 0; i < n; i++) {
+		buf[i] = val;
+	}
 
-    return str;
+	return str;
 }