clarify "pos" in database.c

the list of IDs is stored alphabetically so when accessing the database
with an ID one needs an indirection to get the correct record.
"pos" was not clear. "id_to_pos" is clearer.

common/utils/T/tracer/database.c

@@ -28,7 +28,7 @@ typedef struct {
   int isize;
   group *g;
   int gsize;
-  int *pos;
+  int *id_to_pos;
 } database;
 
 typedef struct {
@@ -326,10 +326,11 @@ void *parse_database(char *filename)
   free(p.name.data);
   free(p.value.data);
 
-  /* setup pos */
-  r->pos = malloc(sizeof(int) * r->isize); if (r->pos == NULL) abort();
+  /* setup id_to_pos */
+  r->id_to_pos = malloc(sizeof(int) * r->isize);
+  if (r->id_to_pos == NULL) abort();
   for (i = 0; i < r->isize; i++)
-    r->pos[r->i[i].id] = i;
+    r->id_to_pos[r->i[i].id] = i;
 
   return r;
 }
@@ -420,7 +421,7 @@ void on_off(void *_d, char *item, int *a, int onoff)
 char *event_name_from_id(void *_database, int id)
 {
   database *d = _database;
-  return d->i[d->pos[id]].name;
+  return d->i[d->id_to_pos[id]].name;
 }
 
 int event_id_from_name(void *_database, char *name)
@@ -442,9 +443,9 @@ database_event_format get_format(void *_database, int event_id)
     abort();
   }
 
-  ret.type = d->i[d->pos[event_id]].arg_type;
-  ret.name = d->i[d->pos[event_id]].arg_name;
-  ret.count = d->i[d->pos[event_id]].asize;
+  ret.type = d->i[d->id_to_pos[event_id]].arg_type;
+  ret.name = d->i[d->id_to_pos[event_id]].arg_name;
+  ret.count = d->i[d->id_to_pos[event_id]].asize;
 
   return ret;
 }