set *errin* isTTY properly in package mode (-p) (#2269)

When running in package mode (-p), as used by xgap, GAP sets *errin* to be the same as *stdin*.  It should at the same time set isTTY to be the same as for *stdin*.

Author: RussWoodroofe

src/system.c

@@ -1935,8 +1935,9 @@ void InitSystem (
     if ( SyWindow ) {
       /*         SyLineEdit   = 1;
                  SyCTRD       = 1; */
-        syBuf[2].fp = fileno(stdin);  syBuf[2].echo = fileno(stdout);
-        syBuf[3].fp = fileno(stdout);
+        syBuf[2].fp = syBuf[0].fp;  syBuf[2].echo = syBuf[0].echo;
+        syBuf[2].isTTY = syBuf[0].isTTY;
+        syBuf[3].fp = syBuf[1].fp;
         syWinPut( 0, "@p", "1." );
     }