osx: fix annoying focus behaviour (@anticat)

https://github.com/Proxmark/proxmark3/pull/689

OS X has a global menu bar and a per app dock icon. Therefore, all
GUI applications launched from a terminal will become focused - even
if they don’t show any windows. Thereby the terminal loses focus.

Since is it very annoying to re-focus the terminal after each proxmark
client launch, this change makes the client unfocusable during launch
and restores the regular behaviour when a window is created.

client/util_darwin.h

@@ -13,5 +13,7 @@
 
 void disableAppNap(const char* reason);
 void enableAppNap();
+void makeUnfocusable();
+void makeFocusable();
 
 #endif