In the endless batch mode (-B), terminate if and when stdout is closed.

That mode is useful to call gstat from other app, however kinda useless
since gstat won't exit and stay running forever when its parent process
has long gone.

MFC after:	2 weeks

usr.sbin/gstat/gstat.c

@@ -517,7 +517,8 @@ main(int argc, char **argv)
 			if (!flag_B)
 				loop = 0;
 			else
-				fflush(stdout);
+				if (fflush(stdout) == EOF)
+					goto out;
 			usleep(flag_I);
 			continue;
 		}
@@ -585,7 +586,7 @@ main(int argc, char **argv)
 			}
 		}
 	}
-
+out:
 	if (!flag_b) {
 		el_end(el);
 		endwin();