py_talloc interface

[tridge at samba.org]

Hi Jelmer,

 > Thanks! This makes the bug less visible, but I think we should also make
 > make the code fail in this situation and fix
 > PyCredentialCacheContainerObject. Things like py_talloc_dealloc should
 > ensure that they are only used with objects that were created using
 > those functions, perhaps using a magic value?

I did consider using talloc_set_name() to a magic value on the
mem_ctx, and have it check that, but I thought there may be users who
are not using the py_talloc allocator for all allocations that end up
at the de-allocator. If you think this shouldn't be allowed then I can
put the check in.

Cheers, Tridge