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.

m-snapshot.h

@@ -313,12 +313,12 @@ static inline void snapshot_mrsw_int_init(snapshot_mrsw_int_t s, size_t n)
   genint_init (s->freeList, n);
   // Get a free buffer and set it as available for readers
   unsigned int w = genint_pop(s->freeList);
-  assert (w != -1U);
+  assert (w != GENINT_ERROR);
   atomic_store(&s->cptTab[w], 1U);
   atomic_init(&s->lastNext, SNAPSHOTI_SPMC_INT_FLAG(w, true));
   // Get working buffer
   s->currentWrite = genint_pop(s->freeList);
-  assert (s->currentWrite != -1U);
+  assert (s->currentWrite != GENINT_ERROR);
   atomic_store(&s->cptTab[s->currentWrite], 1U);
   SNAPSHOTI_SPMC_INT_CONTRACT(s);
 }
@@ -367,7 +367,7 @@ static inline unsigned int snapshot_mrsw_int_write_idx(snapshot_mrsw_int_t s, un
       // If someone else keeps a ref on the buffer, we can't reuse it
       // get another free one.
       idx = genint_pop(s->freeList);
-      assert(idx != -1U);
+      assert(idx != GENINT_ERROR);
     }
     assert (idx < s->n + SNAPSHOTI_SPMC_EXTRA_BUFFER);
     assert (atomic_load(&s->cptTab[idx]) == 0);
@@ -389,7 +389,7 @@ static inline unsigned int snapshot_mrsw_int_write_start(snapshot_mrsw_int_t s)
   SNAPSHOTI_SPMC_INT_CONTRACT(s);
   // Get a new buffer.
   unsigned int idx = genint_pop(s->freeList);
-  assert (idx != -1U);
+  assert (idx != GENINT_ERROR);
   assert (idx < s->n + SNAPSHOTI_SPMC_EXTRA_BUFFER);
   assert (atomic_load(&s->cptTab[idx]) == 0);
   atomic_store(&s->cptTab[idx], 1U);
@@ -583,7 +583,7 @@ static inline void snapshot_mrsw_int_read_end(snapshot_mrsw_int_t s, unsigned in
   {									\
     M_C(name, _mrsw_init)(snap->core, nReader + nWriter -1 );           \
     unsigned int idx = snap->core->core->currentWrite;                  \
-    snap->core->core->currentWrite = -1;                                \
+    snap->core->core->currentWrite = GENINT_ERROR;                      \
     snapshot_mrsw_int_write_end(snap->core->core, idx);                 \
   }									\
                                                                         \