[kernel] Let endpoints get interrupt notifications

- Add a tag field to all endpoint messages, which doubles as a
  notification field
- Add a endpoint_bind_irq syscall to enable an endpoint to listen for
  interrupt notifications. This mechanism needs to change.
- Add a temporary copy of the serial port code to nulldrv, and let it
  take responsibility for COM2

src/include/j6/types.h

@@ -10,16 +10,30 @@ typedef uint64_t j6_koid_t;
 /// Syscalls return status as this type
 typedef uint64_t j6_status_t;
 
-/// Handles are references and capabilities to other objects
-typedef uint32_t j6_handle_t;
-
 /// Some objects have signals, which are a bitmap of 64 possible signals
 typedef uint64_t j6_signal_t;
 
-/// The rights of a handle/capability are a bitmap of 64 possible rights
-typedef uint64_t j6_rights_t;
+/// The first word of IPC messages are the tag. Tags with the high bit
+/// set are reserved for the system.
+typedef uint64_t j6_tag_t;
 
-#define j6_handle_invalid 0xffffffff
+#define j6_tag_system_flag    0x8000000000000000
+
+/// If all high bits except the last 16 are set, then the tag represents
+/// an IRQ.
+#define j6_tag_irq_base       0xffffffffffff0000
+#define j6_tag_is_irq(x)      (((x) & j6_tag_irq_base) == j6_tag_irq_base)
+#define j6_tag_from_irq(x)    ((x) | j6_tag_irq_base)
+#define j6_tag_to_irq(x)      ((x) & ~j6_tag_irq_base)
+
+/// Handles are references and capabilities to other objects. The least
+/// significant 32 bits are an identifier, and the most significant 32
+/// bits are a bitmask of capabilities this handle has on that object.
+typedef uint64_t j6_handle_t;
+
+#define j6_handle_rights_shift 4
+#define j6_handle_id_mask 0xffffffffull
+#define j6_handle_invalid 0xffffffffull
 
 /// A process' initial data structure for communicating with the system
 struct j6_process_init