[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/kernel/serial.cpp

@@ -1,8 +1,12 @@
+#include "kutil/no_construct.h"
 #include "io.h"
 #include "serial.h"
 
-serial_port g_com1(COM1);
-
+// This object is initialized _before_ global constructors are called,
+// so we don't want it to have global constructors at all, lest it
+// overwrite the previous initialization.
+static kutil::no_construct<serial_port> __g_com1_storage;
+serial_port &g_com1 = __g_com1_storage.value;
 
 serial_port::serial_port() :
 	m_port(0)
@@ -12,6 +16,13 @@ serial_port::serial_port() :
 serial_port::serial_port(uint16_t port) :
 	m_port(port)
 {
+	outb(port + 1, 0x00);  // Disable all interrupts
+	outb(port + 3, 0x80);  // Enable the Divisor Latch Access Bit
+	outb(port + 0, 0x01);  // Divisor low bit: 1 (115200 baud)
+	outb(port + 1, 0x00);  // Divisor high bit
+	outb(port + 3, 0x03);  // 8-N-1
+	outb(port + 2, 0xe7);  // Clear and enable FIFO, enable 64 byte, 56 byte trigger
+	outb(port + 4, 0x0b);  // Data terminal ready, Request to send, aux output 2 (irq enable)
 }
 
 bool serial_port::read_ready() { return (inb(m_port + 5) & 0x01) != 0; }