Let 'a' be a default for --dot

Since we support acceptance conditions that are more complex that the generalized-Büchi acceptance we used to support (pre 2.0), it makes more sense to always display the acceptance condition of the automaton. Having to pass a to the print_dot() function or to the --dot option is quite tedious.

In Spot 2.4, we introduced option A as the negation of a, so in Spot 2.6 it should be safe to change print_dot() so that a is now the default. Users who focus on Büchi or generalized-Büchi and do not care about the acceptance condition can start to use A and be backward-compatible down to 2.4.