SF bug #887946, segfault if redirecting directory

Also provide a warning if a directory is passed on the command line.
Add minimal command line test.

Will backport.

Modules/main.c

@@ -359,6 +359,14 @@ Py_Main(int argc, char **argv)
 					}
 				}
 			}
+			{
+				/* XXX: does this work on Win/Win64? (see posix_fstat) */
+				struct stat sb;
+				if (fstat(fileno(fp), &sb) == 0 &&
+				    S_ISDIR(sb.st_mode)) {
+					fprintf(stderr, "%s: warning '%s' is a directory\n", argv[0], filename);
+				}
+			}
 		}
 	}