twa_graph: add support for universal initial states

The only missing point is that the HOA parser cannot deal with multiple
universal initial states, as seen in parseaut.test.

* spot/graph/graph.hh (new_univ_dests): New function, extracted from...
(new_univ_edge): ... this one.
* spot/twa/twagraph.hh (set_univ_init_state): Implement using
new_univ_dests.
* spot/twaalgos/dot.cc, spot/twaalgos/hoa.cc, python/spot/impl.i:
Add support for universal initial states.
* spot/parseaut/parseaut.yy: Add preliminary support for
universal initial states.  Multiple universal initial states
are still not supported.
* tests/core/alternating.test, tests/core/parseaut.test,
tests/python/alternating.py: Adjust tests and exercise this new feature.

tests/core/alternating.test

@@ -24,7 +24,7 @@ set -e
 cat >alt.hoa <<EOF
 HOA: v1.1
 States: 7
-Start: 0
+Start: 0&2
 acc-name: co-Buchi
 Acceptance: 1 Fin(0)
 AP: 2 "b" "a"
@@ -62,14 +62,17 @@ digraph G {
   label="Fin(⓿)"
   labelloc="t"
   I [label="", style=invis, width=0]
-  I -> 0
+  I -> -11 [dir=none]
+    -11 [label=<>,width=0,height=0,shape=none]
+    -11 -> 0
+    -11 -> 2
   0 [label="((((a) U (b)) && GF(b)) && FG(a))"]
   0 -> -1 [label="b", dir=none]
-    -1[label=<>,width=0,height=0,shape=none]
+    -1 [label=<>,width=0,height=0,shape=none]
     -1 -> 3
     -1 -> 1
   0 -> -4 [label="a & !b", style=bold, color="#F15854", dir=none]
-    -4[label=<>,width=0,height=0,shape=none]
+    -4 [label=<>,width=0,height=0,shape=none]
     -4 -> 5 [style=bold, color="#F15854"]
     -4 -> 3 [style=bold, color="#F15854"]
     -4 -> 1 [style=bold, color="#F15854"]
@@ -81,7 +84,7 @@ digraph G {
   3 [label="GF(b)"]
   3 -> 3 [label="b"]
   3 -> -8 [label="!b", style=bold, color="#FAA43A", dir=none]
-    -8[label=<>,width=0,height=0,shape=none]
+    -8 [label=<>,width=0,height=0,shape=none]
     -8 -> 4 [style=bold, color="#FAA43A"]
     -8 -> 3 [style=bold, color="#FAA43A"]
   4 [label="F(b)\n⓿"]

tests/core/parseaut.test

@@ -1921,8 +1921,7 @@ State: 11 "t"
 EOF
 
 expecterr input <<EOF
-input:4.8-12: alternation is not yet supported for initial states
-autfilt: failed to read automaton from input
+input:4.1-12: alternating automata only support a single initial state
 EOF
 
 # Some alternation errors