Make TALLOC_CTX and talloc_chunk private to talloc.c.

Make TALLOC_CTX and talloc_chunk private to talloc.c.

Add a global singly-linked list of all active talloc pools, so that we
can eventually show how much memory is used for different purposes.
This also gives a check that pools are not being doubly freed.

talloc_init_named now handle a NULL name properly (ie does nothing)

Add accessor talloc_pool_name().