Basic support the backspace key.

This doesn't mean we get any line editing just yet. But the keyboard device
now recognizes the backspace key, and the console device knows what to do
with the backspace characters.

Kernel/Console.cpp

@@ -303,6 +303,14 @@ void Console::putChar(char ch)
     case '\033':
         m_escState = ExpectBracket;
         return;
+    case 8: // Backspace
+        if (m_cursorColumn) {
+            --m_cursorColumn;\
+            vga_set_cursor(m_cursorRow, m_cursorColumn);
+            vga_putch_at(m_cursorRow, m_cursorColumn, ' ');
+            return;
+        }
+        break;
     case '\n':
         scrollup();
         vga_set_cursor(m_cursorRow, m_cursorColumn);