[kernel] Add handle_clone syscall

Added the handle_clone syscall which allows for cloning a handle with
a subset of the original handle's capabilities.

Related changes:

- srv.init now calls handle_clone on its system handle, and load_program
  was changed to allow this second system handle to be passed to loaded
  programs instead. However, as drv.uart is still a driver AND a log
  reader, this new handle is not actually passed yet.
- The definition parser was using a set for the cap list, which meant
  the order (and thus values) of caps was not static.
- Some code in objects/handle.h was made more explicit about what bits
  meant what.

src/kernel/objects/handle.h

@@ -9,11 +9,18 @@ namespace obj {
 
 struct handle
 {
+    constexpr static uint64_t id_mask   = 0x00000000ffffffff;
+    constexpr static uint64_t cap_mask  = 0x00ffffff00000000;
+    constexpr static uint64_t type_mask = 0xff00000000000000;
+
+    constexpr static unsigned cap_shift  = 32;
+    constexpr static unsigned type_shift = 56;
+
     // A j6_handle_t is an id in the low 32 bits, caps in bits 32-55, and type in 56-63
     static inline j6_handle_t make_id(j6_handle_t id, j6_cap_t caps, kobject *obj) {
         return (id & 0xffffffffull) |
-            static_cast<j6_handle_t>(caps) << 32 |
-            static_cast<j6_handle_t>(obj ? obj->get_type() : kobject::type::none) << 56;
+            ((static_cast<j6_handle_t>(caps) << cap_shift) & cap_mask) |
+            static_cast<j6_handle_t>(obj ? obj->get_type() : kobject::type::none) << type_shift;
     }
 
     inline handle(j6_handle_t in_id, kobject *in_obj, j6_cap_t caps) :
@@ -50,7 +57,7 @@ struct handle
         if (object) object->handle_release();
     }
 
-    inline j6_cap_t caps() const { return id >> 32; }
+    inline j6_cap_t caps() const { return (id & cap_mask) >> cap_shift; }
 
     inline bool has_cap(j6_cap_t test) const {
         return (caps() & test) == test;

src/kernel/syscalls/handle.cpp

@@ -2,6 +2,7 @@
 #include <j6/types.h>
 
 #include "objects/process.h"
+#include "syscalls/helpers.h"
 
 using namespace obj;
 
@@ -24,4 +25,19 @@ handle_list(j6_handle_t *handles, size_t *handles_len)
     return j6_status_ok;
 }
 
+j6_status_t
+handle_clone(j6_handle_t orig, j6_handle_t *clone, uint32_t mask)
+{
+    handle *orig_handle = get_handle<kobject>(orig);
+    if (!orig_handle)
+        return j6_err_invalid_arg;
+
+    process &p = process::current();
+    *clone = p.add_handle(
+            orig_handle->object,
+            orig_handle->caps() & mask);
+
+    return j6_status_ok;
+}
+
 } // namespace syscalls