Add first work on keyboard support

Mainly the switch_read function, which had necessitated some work done
in the vm_screen area

include/apple2.kb.h

@@ -0,0 +1,7 @@
+#ifndef _APPLE2_KB_H_
+#define _APPLE2_KB_H_
+
+#include "apple2.h"
+#include "vm_segment.h"
+
+#endif

src/apple2.kb.c

@@ -0,0 +1,53 @@
+/*
+ * apple2.kb.c
+ */
+
+#include "apple2.kb.h"
+
+SEGMENT_READER(apple2_kb_switch_read)
+{
+    apple2 *mach = (apple2 *)_mach;
+    char ch = '\0';
+
+    switch (addr) {
+        case 0xC000:
+            // This _also_ returns the 7 bit high if the strobe is still
+            // set. Once you read from $C010, this bit will be cleared
+            // in future reads--until another key is pressed.
+            if (mach->screen) {
+                ch = vm_screen_last_key(mach->screen);
+
+                // If the strobe is set, we need to set the 7 bit on the
+                // return value. (NOTE: Apple II can only handle 7-bit
+                // ASCII, so the highest bit (bit 7, if counting from
+                // zero) is always low in regards to ASCII
+                // representation; it can _only_ be high if the strobe
+                // is set.
+                if (mach->strobe) {
+                    return ch | 0x80;
+                }
+            }
+
+            // If we have no screen, we'll just end up returning
+            // whatever is in ch already (NUL).
+            return ch;
+            
+        case 0xC010:
+            // First, we need to clear the keyboard strobe.
+            mach->strobe = false;
+
+            if (mach->screen) {
+                // Now we need to return whether any key was pressed, but
+                // we'll have to ask the vm_screen subsystem. We just want
+                // to return with the 7 bit high if it is pressed, and all
+                // zeroes if not.
+                return vm_screen_key_pressed(mach->screen)
+                    ? 0x80
+                    : 0x00;
+            }
+
+            // We have no keyboard, so we basically can have no key
+            // pressed down.
+            return 0;
+    }
+}