Kernel: Add a beep() syscall that beeps the PC speaker.

Hook this up in Terminal so that the '\a' character generates a beep.
Finally emit an '\a' character in the shell line editing code when
backspacing at the start of the line.

Kernel/Devices/PCSpeaker.cpp

@@ -0,0 +1,20 @@
+#include <Kernel/Devices/PCSpeaker.h>
+#include <Kernel/i8253.h>
+#include <Kernel/IO.h>
+#include <Kernel/i386.h>
+
+void PCSpeaker::tone_on(int frequency)
+{
+    IO::out8(PIT_CTL, TIMER2_SELECT | WRITE_WORD | MODE_SQUARE_WAVE);
+    word timer_reload = BASE_FREQUENCY / frequency;
+
+    IO::out8(TIMER2_CTL, LSB(timer_reload));
+    IO::out8(TIMER2_CTL, MSB(timer_reload));
+
+    IO::out8(0x61, IO::in8(0x61) | 3);
+}
+
+void PCSpeaker::tone_off()
+{
+    IO::out8(0x61, IO::in8(0x61) & ~3);
+}

Kernel/Devices/PCSpeaker.h

@@ -0,0 +1,7 @@
+#pragma once
+
+class PCSpeaker {
+public:
+    static void tone_on(int frequency);
+    static void tone_off();
+};