DP: Add function to get be_ctx directly from dp_client Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>

DP: Introduce new interface for backend Terminology: * Backend: Implemenation of domain * Data Provider: interface between backend and responders * Module: ldap/ipa/ad/... dlopened library that implements dp interface * Target: id/autofs/sudo/... functionality of module Benefits over current code: * data provider is a black box completely separated from backend * method handlers are just simple tevent requests on backend side * no need of spy on be_client * simplified and error proof adding of new responders * simplified adding of new methods * reply to D-Bus message is completely handled by DP code * each target can have several methods defined * properties can be added on objects * each method can have output parameters * modules now support constructor * improved debugging * clear memory hierarchy * ability to chain requests * type safe private data Reviewed-by: Sumit Bose <sbose@redhat.com> Reviewed-by: Jakub Hrozek <jhrozek@redhat.com> Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>