dsdb: Use customary variable names for the audit private context

dsdb: Use customary variable names for the audit private context

The variable name "ac" typically implies the async context, and the long-life
private context is normally denoted private, not context.  This aligns better
with other modules.

talloc_get_type_abort() is now also used.

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Gary Lockyer <gary@catalyst.net.nz>