[kernel] Make stdout channel available to processes

The "fake" stdout channel is now being passed in the new j6_process_init
structure to processes, and nulldrv now uses it to print a message to
the console.

src/include/j6/types.h

@@ -20,3 +20,10 @@ typedef uint64_t j6_signal_t;
 typedef uint64_t j6_rights_t;
 
 #define j6_handle_invalid 0xffffffff
+
+/// A process' initial data structure for communicating with the system
+struct j6_process_init
+{
+	j6_handle_t input;
+	j6_handle_t output;
+};