Start adding some debugging features to talloc based on Samba's

Start adding some debugging features to talloc based on Samba's
ancient mem_man.c:

Each TALLOC_CTX now has a field to store its purpose, to aid in
tracking down memory bloat.  A new call talloc_init_named() should be
used instead of talloc_init() so that this is set.

Added talloc_vasprintf to be called by varargs functions.