Fix reading of '$if GAP' part in ~/.inputrc

Make sure that
  rl_readline_name = "GAP"
is set before the
  rl_initialize();
initialization of readline. Otherwise GAP specific settings in
~/.inputrc under

$if GAP
  ...
$endif

are not read at startup.

src/system.c

@@ -1155,6 +1155,7 @@ void InitSystem (
     InitSysFiles();
 
 #ifdef HAVE_LIBREADLINE
+    rl_readline_name = "GAP";
     rl_initialize ();
 #endif