Clean up interface if (internal) genint:

it returns at most an 2048, so an unsigned int is more enought
(and internally it was already used!).
So transform the interface to take as input unsigned int.

Fix maximum size supported by the genint package.

Add proper constant used for error, and use it in the code.

tests/test-mgenint.c

@@ -40,8 +40,8 @@ static void test(size_t n)
       assert (i == j);
     }
     i = genint_pop(s);
-    assert (i == -1UL);
-    assert (genint_pop(s) == -1UL);
+    assert (i == GENINT_ERROR);
+    assert (genint_pop(s) == GENINT_ERROR);
     
     for(size_t j = 0; j< n ; j++) {
       genint_push(s, (j+k) % n );
@@ -63,14 +63,14 @@ static void conso(void *p)
     unsigned int tab[4];
     for(int j= 0; j < 4; j++) {
       tab[j] = genint_pop(global);
-      if (tab[j] != -1U) {
+      if (tab[j] != GENINT_ERROR) {
         assert (tab[j] < n);
         assert (atomic_load(&tab_g[tab[j]]) == false);
         atomic_store(&tab_g[tab[j]], true);
       }
     }
     for(int j = 0; j < 4; j++) {
-      if (tab[j] != -1U) {
+      if (tab[j] != GENINT_ERROR) {
         assert (tab[j] < n);
         assert (atomic_load(&tab_g[tab[j]]) == true);
         atomic_store(&tab_g[tab[j]], false);