values.

Before this change states were numbered using positive values and
SCCs using negative values.  That meant the user had to work with
negative values.  With this changes, the nonnegative values used
to label SCCs can also directly be used as index in the scc_map_.

* src/tgbaalgos/scc.hh (scc_map::scc_of_state,
scc_map::cond_set_of, scc_map::acc_set_of, scc_map::states_of,
scc_map::initial, scc_map::scc_type, scc_map::succ,
scc_map::accepting): Adjust prototypes to take or return unsigned
arguments.
* src/tgbaalgos/scc.cc: Adjust prototypes of the above functions.
(scc_map::build_map, scc_map::relabel_component): Number states
using negative values, and SCCs using nonnegative values.
(dump_scc_dot): Adjust to use nonnegative values.