Comms refactor (prerequisite of libproxmark work) (#371)

* Refactor the comms code only from PR#346, without comms_globals.h.
* OSX: Add note for example serial port

client/cmdparser.c

@@ -14,6 +14,7 @@
 #include "ui.h"
 #include "cmdparser.h"
 #include "proxmark3.h"
+#include "comms.h"
 
 
 void CmdsHelp(const command_t Commands[])
@@ -23,7 +24,7 @@ void CmdsHelp(const command_t Commands[])
   int i = 0;
   while (Commands[i].Name)
   {
-    if (!offline || Commands[i].Offline)
+    if (!IsOffline() || Commands[i].Offline)
        PrintAndLog("%-16s %s", Commands[i].Name, Commands[i].Help);
     ++i;
   }