Define the type of marking flags as unsigned int.
Define the type of marking flags as unsigned int.

We could define it as _Bool since we are going to use C99, but unsigned
int is more portable (even in C89!) and extensible (when we decide to
use tri-color marking GC.)

Signed-off-by: OGINO Masanori <c49462fef669560e618607913e6c283f7627123c@gmail.com>

omasanori/picrin,omasanori/picrin,koba-e964/picrin,picrin-scheme/picrin,koba-e964/picrin,dcurrie/picrin,ktakashi/picrin,leavesbnw/picrin,koba-e964/picrin,picrin-scheme/picrin,ktakashi/picrin,ktakashi/picrin,leavesbnw/picrin,leavesbnw/picrin,dcurrie/picrin