Added means to search for threads in other containers

Thread ids now contain their container ids in the top 2 nibbles.
Threads on other containers can be addressed by changing those
two nibbles. The addressing of inter-container threads are
subject to capabilities.

include/l4/generic/container.h

@@ -107,6 +107,7 @@ int container_init_pagers(struct kernel_resources *kres,
 			  pgd_table_t *current_pgd);
 
 int init_containers(struct kernel_resources *kres);
+struct container *container_find(struct kernel_resources *kres, l4id_t cid);
 
 #endif /* __CONTAINER_H__ */