[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/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