Ensured that the local_socket constructor sets the close-on-exec flag.
Ensured that local_socket::pair() sets the close-on-exec flag.
Implemented the descriptor#cloexec() methods.
Ensured that copying a descriptor copies the close-on-exec flag state.
Implemented the descriptor#dup() method.
Implemented the descriptor#release() method.
Simplified close-on-exec handling in the directory::iterator constructor.