Refactor commandline handling

- instead of having the list of commandline arguments, their handlers
   and their help strings in different places, unify them in a single
   array
 - this allows to only change one piece of code when adding new
   arguments

cmd_whoami.c

@@ -1,5 +1,6 @@
 /*
  * Copyright (C) 2013 Bryan Christ <bryan.christ@mediafire.com>
+ *               2014 Johannes Schauer <j.schauer@email.de>
  *
  * This program is free software; you can redistribute it and/or modify it
  * under the terms of the GNU General Public License version 2, as published by
@@ -27,10 +28,15 @@
 #include "command.h"
 
 int
-mfshell_cmd_whoami(mfshell_t *mfshell)
+mfshell_cmd_whoami(mfshell_t *mfshell, int argc, char **argv)
 {
     int retval;
 
+    if (argc != 1) {
+        fprintf(stderr, "Invalid number of arguments\n");
+        return -1;
+    }
+
     retval = mfshell->user_get_info(mfshell);
     mfshell->update_secret_key(mfshell);