Set timestamp automatically on write

Do it in do_write().  Remove the setting of timestamps elsewhere.

This fixes empdump to set timestamps to the current time instead of
zero on import.

src/lib/commands/real.c

@@ -89,7 +89,6 @@ real(void)
 	realm.r_xh = abs.hx - 1;
 	realm.r_yl = abs.ly;
 	realm.r_yh = abs.hy - 1;
-	realm.r_timestamp = time(NULL);
 	putrealm(&realm);
 	list_realm(curr, natp);
     }