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.

LibC/unistd.cpp

@@ -475,4 +475,9 @@ int donate(int tid)
     __RETURN_WITH_ERRNO(rc, rc, -1);
 }
 
+void beep()
+{
+    syscall(SC_beep);
+}
+
 }