regex: re-introduce nfa_accept states.

The nfa_accept state label is re-introduced.  This state type
has the same representation as nfa_empty; essentially, this
replaces the flag. This makes the state type smaller, and we
don't have to access the flag to tell if a state is an
acceptance state.

* regex.c (nfa_kind_t): New enum label, nfa_accept.
(struct nfa_state_empty): Member accept removed.
(nfa_accept_state_p): Macro tests only for nfa_accept type.
(nfa_empty_state_p): New macro.
(nfa_state_accept): Set type of new state to nfa_accept;
do not set accept flag.
(nfa_state_empty): Do not set accept flag.
(nfa_state_empty_convert): Do not clear accept flag.
(nfa_map_states): Handle nfa_accept in switch, in the
same case as nfa_empty.
(nfa_thread_epsilons): Don't test for accept state in
nfa_empty case; it would be always false now.  Add nfa_accept
case to switch which only arranges for a traversal of the two
transitions. (Though these are expected to be null at the
stage of the graph when this function is applied).
(nfa_fold_accept): Switch type to nfa_accept rather than
setting accept flag.
(nfa_closure, nfa_move_closure): Use new macro for testing
whether a state is empty.

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