Add memory limiting capability to talloc

Add memory limiting capability to talloc

By calling talloc_set_memlimit() we can now set a max memory limit
for a whole talloc hierarchy.
ANy attempt to allocate memory beyond the max allowed for the whole
hierarchy wil cause an allocation failure.

Stealing memory correctly accounts for used memory in the old and the new
hierarchy but exceeding the memory limit in the new parent will not cause
a failure.