lib: talloc: Add two information / debug functions to talloc pool functionality

Currently use of memory within a talloc pool is somewhat opaque.

Tallocing a new object where the parent is a talloc pool may or may not result in memory being allocated from the pool memory.

By adding these two functions the caller can both check the space left in the pool and test whether an object has been allocated from pool memory. This adds the ability for improved use of talloc pools with the caller having visibility of the state of the pool and the success / failure to allocate from pool memory.