Add some extra macOS Caps Lock stuff.

Add specific messaging on startup when IOHidManagerOpen returns the
value that implies b2 hasn't been granted keyboard input monitoring
privileges. Include link to docs on github.

Add note to docs about running from dmg directly possibly producing odd
behaviour.

Fix #345.

doc/Installing-on-OSX.md

@@ -17,7 +17,8 @@ You don't need to download any of the other files; the dmg contains
 everything required.
 
 Open the dmg, drag b2 to your Applications folder, and run it from
-there.
+there. (Running it from the dmg directly is unsupported, and it may
+not work. I've had reports of odd behaviour when attempting it!)
 
 ## macOS files missing from releases page?
 

src/b2/b2.cpp

@@ -539,13 +539,19 @@ static void InitHIDCallback(Messages *msg) {
             goto cleanup;
         }
     }
-    
+
     IOHIDManagerSetDeviceMatchingMultiple(g_hid_manager, matches);
     IOHIDManagerRegisterInputValueCallback(g_hid_manager, &HIDCallback, g_hid_manager);
     IOHIDManagerScheduleWithRunLoop(g_hid_manager, CFRunLoopGetMain(), kCFRunLoopDefaultMode);
     ior = IOHIDManagerOpen(g_hid_manager, kIOHIDOptionsTypeNone);
     if (ior != kIOReturnSuccess) {
-        msg->i.f("InitHIDCallback: IOHIDManagerOpen returned: %" PRIu32 " (0x%" PRIx32 ")\n", (uint32_t)ior, (uint32_t)ior);
+        if (ior == kIOReturnNotPermitted) {
+            msg->i.f("b2 has not been granted permission to monitor keyboard input.\n");
+            msg->i.f("The emulated BBC will not respond properly to the Caps Lock key!\n");
+            msg->i.f("For more info, please see https://github.com/tom-seddon/b2/blob/master/doc/Installing-on-OSX.md\n");
+        } else {
+            msg->i.f("InitHIDCallback: IOHIDManagerOpen returned: %" PRIu32 " (0x%" PRIx32 ")\n", (uint32_t)ior, (uint32_t)ior);
+        }
         goto cleanup;
     }