Implement talloc_pool()

Implement talloc_pool()

A talloc pool is a chunk of memory that can be used as a context for further
talloc calls. Allocations with the pool as the parent just chew from that
memory by incrementing a pointer. If the talloc pool is full, then we fall back
to the normal system-level malloc(3) to get memory.

The use case for talloc pools is the transient memory that is used for handling
a single SMB request. Incrementing a pointer will be way faster than any malloc
implementation.

There is a downside of this: If you use talloc_steal() to move something out of
the pool, the whole pool memory is kept around until the last object inside the
pool is freed. So if you talloc_free() the pool, it might happen that the
memory is freed later. So don't hang anything off a talloc pool that should
live long.

Volker
(This used to be commit 287e29d988813007eeebc0c2bef3b46ab8bedee9)